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Cellular RNAs: Varied RolesJian Gu, Baylor College of Medicine, Houston, Texas, USA

Ram Reddy, Baylor College of Medicine, Houston, Texas, USA

In the cell, there are three major types of RNA directly involved in protein synthesis.In addition, many other cellular RNAs also play important functions.

Introduction

RNA is a molecule of many facets and subtleties,participating in almost all macromolecular processes.The ‘central dogma’ states that genetic informationflows from DNA to RNA and then to protein: DNA!RNA!Protein. There are three major types of cellularRNA directly involved in protein synthesis: messengerRNA (mRNA), ribosomal RNA (rRNA) and transferRNA (tRNA). In addition, there are many other RNAs

playing varied roles in thecell. In this article, we first brieflyreview the general aspects of three major cellular RNAsinvolved in protein synthesis, then discuss some otherimportant RNA species. Organellar RNAs, i.e. eukaryoticmitochondrial and plant chloroplast RNAs, many of which are transcribed from their independent genomes bydistinct RNA polymerases, are covered elsewhere.

Overview of mRNA, rRNA and tRNA inDifferent Species

RNAs are transcribed from DNA by RNApolymerases

In prokaryotic cells, all cellular RNAs are synthesized by asingle RNA polymerase. In contrast, there are three RNApolymerases in eukaryotes: RNA polymerases I, II and III,which differ in template specificity, localization andsusceptibility to inhibitors. RNA polymerase I (pol I)synthesizes rRNA in nucleoli; RNA pol II makes mostlyprecursor mRNAs and some small RNAs; RNA pol IIImakes tRNA, 5S rRNA and a fewother small RNAs. Bothpol II and pol IIIare localizedin the nucleoplasm.All ofthe

RNA polymerases are composed of several proteinsubunits.

Messenger RNA (mRNA)

mRNA is the template carrying the genetic message fromthe gene to the ribosomal factories for protein synthesis.mRNAs are heterogeneous in size, ranging from hundredsto thousands of nucleotides. In prokaryotes, geneticinformation in DNA is colinear with the specified product,

so newly transcribed mRNAs are useddirectly as templatefor translation. In eukaryotes, however, a much morcomplex process occurs to produce mature mRNAtemplates for translation.

5’ Capping

Most eukaryotic mRNAs contain a 7-methylguanosinresidue (called ‘cap’) attached to the terminal residue oinitial transcript through a 5’ ppp 5’ linkage. This castructure is required for translation initiation and contributes to mRNA stability and export. ProkaryotimRNAs do not contain a 5’ cap structure, but the initiatiocodon is preceded by a short stretch of purine-ricsequence (the Shine–Dalgarno sequence), which facilitatetranslation initiation.

3’ Polyadenylation

The 3’ end of both prokaryotic and eukaryotic mRNAs arpolyadenylated, but there are fundamental differencebetween them. Most eukaryotic mRNAs contain 50–20

adenylic acid residues at their 3’ ends, but the poly(A) tractof prokaryotic mRNA are generally shorter, ranging from15 to 60 adenylic acid residues and are associated with onl2–60% of the molecules of a given mRNA species. Theukaryotic polyadenylation machinery recognizes a specific consensus near the 3’ end, whereas the sites opolyadenylation of prokaryotic mRNA are diverse, anthe reaction does not require a consensus sequence. Thpoly(A) tail functions in mRNA turnover and also imRNA translation.

Article Contents

Introductory article

. Introduction

. Overview of mRNA, rRNA and tRNA in Different

Species

. Small Nuclear RNA (snRNA)

. Small Nucleolar RNA (snoRNA)

. Small Cytoplasmic RNAs (scRNAs)

. MRP RNA

. RNase P RNA

. The 7SL RNA Component of the Signal Recognition

Particle

. 4.5S RNA

. Alu transcripts

. Telomerase RNAs

. RNA Primers for Okazaki Fragments

. Non-protein-coding mRNAs

.

Maternal mRNAs in Development

ENCYCLOPEDIA OF LIFE SCIENCES © 2001, John Wiley & Sons, Ltd. www.els.net



Splicing of intervening sequences (introns)

Genes in eukaryotes are often interrupted by interveningsequences called introns, which do not code for proteins.The DNA sequences are transcribed with no discrimina-tion between introns and the coding ‘exon’ regions,therefore the primary transcript (pre-mRNA) is litteredwith segments of genetic nonsense. Consequently, mostprotein-coding transcripts must be processed to removethese introns before protein expression can occur. Theprocess by which introns are removed and the flankingexons are stitched back together is called RNA splicing.Splicing occurs within a large ribonucleoprotein (RNP)complex called the spliceosome. A unique collection of cellular RNAs, called small nuclear RNAs (snRNAs), arecritical for pre-mRNA splicing.

Other posttranscriptional events

In addition to the above common processing events, thereare some other events which occur in a few individual

mRNAs, including nucleotide modifications and RNAediting.

Transfer RNA (tRNA)

tRNAs serve a dual function in protein synthesis: theycontain a site (3’ CCA) for attachment of the amino acidand another site (anticodon) that interacts with themRNA. Each amino acid is attached to a specific tRNA,and usually there are several tRNA species for each aminoacid.

The length of known tRNAs varies from 72 to 95nucleotides, most of which are 76 nucleotides long and

form the typical tRNA cloverleaf secondary structure. Thesite of amino acid attachment is always the 3’ endadenosine residue, which is part of the constant 3’ CCAOHterminus; the anticodon residues usually are nucleotides34, 35 and 36.

All tRNAs are derived by processing larger precursormolecules containing stretches of additional nucleotides atboth ends. In addition to the removal of the 5 ’ and 3’ extrasequences, some other events may occur before thematuration of tRNAs, notably, the occurrence of a highpercentage of modified nucleotides. A small population of eukaryotic tRNA species, especially in yeast, and a fewbacterial tRNAs contain introns.

Ribosomal RNA (rRNA)

Protein synthesis takes place on the ribosome, which is alarge RNP complex containing by weight about two-thirdsRNA and one-third protein. Ribosomes from all sourcesconsist of small and large subunits. Figure 1 shows acomparison of the prokaryotic and eukaryotic ribosomes.

Both the large subunit and small subunit rRNAs arefolded into defined structures with many short duplex

regions, which provide a structural scaffold for proteibinding. There are many interactions in the ribosomeamong rRNAs, mRNA and tRNA. Recent advances iribosome research provided convincing evidence tharRNAs, rather than ribosomal proteins, play a centrarole in catalysing the formation of peptide bonds.

In all organisms, rRNAs are transcribed as largprecursors containing structural gene products flankeby extra sequences. In prokaryotes like Escherichia coli ,ththree types of rRNA are transcribed as one long RNA

molecule, which is then processed by nucleolytic cleavagto release full-length, mature rRNAs. Furthermore, durinthe maturation process, the base and ribose modificationfound in mature 16S and 23S rRNA are generated. Thspecificity of the initial cleavage sites depends on the abilitof pre-rRNA to form stem structures involving sequenceflanking both the 16S and 23S rRNAs.

Eukaryotic rRNAs undergo a similar processing pathway but with more complexity. The 18S, 5.8S and 28S (25in yeast) rRNAs are initially transcribed as a single largprecursor molecule by RNA polymerase I and subsequently processed through a series of cleavage reactioninto the mature species. In addition, the primary rRNA

transcript also undergoes methylation and pseudouridation on 18S, 5.8S and 28S rRNA. What is most unique teukaryotes is the participation of small nucleolar RNA(snoRNAs) as guide molecules in the accurate processinand modification of rRNAs. In addition, unlike prokaryotes, eukaryotic 5S rRNA is transcribed separately bRNA polymerase III with little posttranscriptional procesing and modification.

70S ribosome30S subunit

50S subunit

34 Proteins

2 RNAs

5S rRNA

23S rRNA21 Proteins

1 RNA (16S rRNA)

(a)

80S ribosome40S subunit

60S subunit

~49 Proteins

3 RNAs

5S rRNA

5.8S rRN~33 Proteins

1 RNA (18S rRNA)

(b)

28S rRNA

Figure 1 A comparison of the prokaryotic and eukaryotic ribosomalRNAs. Thelargesubunit (LSU) is shown in blue andthe small subunit (SSU

is shown inyellow. (a) In prokaryotes there are three types of rRNA: 5S an23SrRNAsin LSU, 16SrRNAin SSU. (b)In eukaryotes there arefourtypeso

rRNA: 5S, 5.8S and 28S rRNAs in LSU, 18S rRNA in SSU.

Cellular RNAs: Varied Roles

2



Small Nuclear RNA (snRNA)

The nuclei of all eukaryotic cells contain a variety of smallRNA molecules (lengths from 60 to 300, longer in yeast),referred to as small nuclear RNAs (snRNAs). Many of these snRNAs are rich in uridylic acid and were designated‘U’ snRNAs. Each snRNA is tightly associated with a

number of proteins and forms small nuclear ribonucleo-proteins (snRNPs). The snRNPs play distinct and criticalroles in many cellular functions.

The precise removal of introns from pre-mRNA is acritical step in gene expression in all eukaryotic cells. Itoccurs via a two-step transesterificationpathway occurringin spliceosomes (Figure2).Inthefirststep,the2’hydroxyl of a conserved, intronic adenosine (branch site) attacks the 5’splice site, producing a free 5’ exon and a branched species.In the second step, the 3’ hydroxyl of the 5’ free exonattacks the 3’ splice site, yielding ligated exons and a lariatintron.

The vast majority of introns have GU and AG

dinucleotides at their 5’ and 3’ ends, respectively. The fivemost abundant snRNPs, U1, U2, U4, U5 and U6 snRNP,are at the heart of the spliceosome.By formingvarious basepairs with the consensus sequence of the intron and amongthemselves, these snRNAs recognize the intron, align thetwo flanking exons, and mayactually play the catalytic rolein splicing. The spliceosome assembles and rearranges in ahighly ordered and stepwise manner (Figure 2). Briefly, theassembly beginswith the association of U1 snRNP with thepre-mRNA 5’ splice site, subsequently, the U2 snRNPbinds to the branch site, and then the U4/U6/U5 triplesnRNPs join in. Finally, a complicated and dynamicreconstruction process occurs, U1 and U4 snRNPs are

destabilized and released, and the spliceosome is activatedfor the two steps of catalysis as mentioned above.

A few pre-mRNA introns possess termini AU-ACinstead of the canonical GU-AG consensus. Most of theseminor introns are processed in a minor spliceosome, whichis formed by a different set of snRNPs, U11, U12, U4atac,U6atac and U5. U11, U12, U4atac and U6atac are all low-abundance snRNAs and they function in a manneranalogous to U1, U2, U4 and U6 in the splicing of canonical introns. U5 snRNP is the commonplayerin bothtypes of spliceosomes.

There are many other low-abundance U snRNAs. U7snRNA, is involved in the 3’ end formation of some histone

mRNAs. The functions of some abundant snRNAs (e.g.7SK) remain elusive.

Small Nucleolar RNA (snoRNA)

In eukaryotic cells, the nucleolus is a specialized structurefor the biogenesis of ribosomes. In addition to the rRNAs,the nucleolus contains a multitude of discrete small

nucleolar RNAs (snoRNAs). More than 100 distinc

snoRNA sequences have been identified in vertebrateand yeast. In yeast, most of the snoRNAs are transcribefrom independent genes using their own promoterHowever, the majority of mammalian snoRNAs arprocessed from introns of pre-mRNAs. These snoRNAare responsible not only for orchestrating the cleavagevents that cut the long pre-rRNA into 18S, 5.8S and 28Sbut also for determining the specific sites for modificationVertebrate rRNAs contain approximately 105 methylatesugars, 95 pseudouridines, and 10 methylated base

U2

U2

U4 U6

U1 U2

GUexon1 A AG exon2

U5

U5U4 U6

U1

U2 A AG exon2

G U e x o n 1

U5U6

AG exon2

G U e x o n 1

U1

U4

A

1st Step

U5

U6

AG exon2

G U

e x o n 1

A

2nd Step

AG

G U

A

exon2exon1 +

Figure 2 A simplified view of the spliceosome assembly andrearrangement.U1 snRNPbindsto the5’ splice site,U2 subsequently binto thebranchsite andthenU4/U5/U6triple snRNPs join in.After a dynam

rearrangement, U1 and U4 are destabilized, and the spliceosome isactivated for the two steps of cleavage – ligation event.




whereas yeast rRNAs have about half as many modifica-tions. In prokaryotic rRNAs, there are even fewermodifications and it is believed that site-specific enzymesare responsible for these modifications. In contrast, ratherthan developing a specific enzyme for each modifiednucleotide, eukaryotes evolved a unique mechanism forsite-specific modification using probably a very limited

pool of modifying enzymes. snoRNAs exhibit extensive orshort complementarity to the rRNA sequence flanking thenucleotide to be modified and directs either sugarmethylation or pseudouridation.

All known snoRNAs, except for the mitochondrialribosomal protein (MRP) RNA, can be simply classifiedinto two large families. One family is defined by conservedboxes C and D and the other by a consensus ACA tripletpositioned three nucleotides before the 3’ end of the RNA.U3 snoRNA is the first snoRNA to be identified and themost abundant snoRNA. Phylogenetic comparison of U3snoRNAs from various species revealed conserved se-quence elements called boxes C (UGAUGA) and D

(CUGA), which were later found to be present in manysnoRNAs. All of the C/D box snoRNAs bind to anevolutionarily conserved nucleolar protein, fibrillarin, andfunction in various steps of pre-rRNA maturation. TheU3, U8, U14 and U22 snoRNAs have been shown toparticipate in the processing of rRNAs at various cleavagesteps. The vast majority of box C/D snoRNAs have anextensive sequence complementarity (ranging from 10 to21) to highly conserved regions of rRNAand serve as guidemolecules for site-specific ribose methylation. A model forthe selection of 2’-O-methylated nucleotides in rRNAsequences by interaction with box C/D snoRNAs is shownin Figure3a. Accordingto this model,the RNAdouble helix

formed by thesnoRNA and the rRNA is followed by the D

box of the snoRNA. A nucleotide in the rRNA sequencewhich is located in the snoRNA–rRNA helix opposite tthe fifth nucleotide upstream from the D box of thsnoRNA, is selected for ribose methylation.

The box ACA snoRNAs share a phylogeneticallconserved secondary structure. The ACA snoRNAs folinto two hairpin structures connected by a single-strande

hinge region and followed by a short 3’ tail. The hingregion carries an extra conserved motif, called box H(consensus, AnAnnA). The box ACA snoRNAs lacextensive sequence complementarity to rRNA, but thefunction as guide RNAs in the site-specific pseudouridylation of pre-rRNA via an elegant mechanism (Figure 3b). Ithe5’or 3’hairpin element of the snoRNA, an internal loostructure, called the ‘pseudouridylation pocket’, selects thtarget rRNA sequence by forming two short (3–10 bphelix structures that are separated by two unpaireribosomal nucleotides. The first unpaired nucleotide ithe selected rRNA sequence (in a 5’ to 3’ orientation) is uridine residue that is converted into pseudouridine.

Small Cytoplasmic RNAs (scRNAs)

In a broad sense, scRNAs are defined as all the smacytoplasmic RNAs that are not directly involved in proteisynthesis. Except for SRP RNAand Alutranscripts, whicwill be addressed separately, very few scRNAs have beesequenced. Four different scRNAs designated hY1, hY3hY4 and hY5 accumulate in human cells as RNPassociated with the Ro proteins. Ro scRNPs are frequentlrecognized by autoantibodies, especially those found i

patients with systemic lupus erythematosus. Although th

snoRNA 5’ 3’

AG

U AGU AG

UC

5’ rRNA

Box C Box D

2 ’ – O

– M e –

3’

(a)

snoRNA 5’ 3’

Box H

3’

(b)

N Ψ

ANANNA

5’ rRNA

N Ψ

ACA

Box ACA

Figure 3 Schematic representation of box C/D and box ACA snoRNAs in directing 2’-O-methylation and pseudouridine (C) formation. (a) Box C/D

snoRNA directs 2’-O-methylation. (b) Box ACA snoRNA directs pseudouridine formation. Each box ACA snoRNA may contain one or both of the internloop structure, called ‘pseudouridylation pocket’. Modified from Tollervey D and Kiss T (1997) Current Opinion in Cell Biology 9: 337 – 342.


4



function of Ro RNPs is unknown, their evolutionaryconservation and their involvement in human pathologicconditions suggest an important biological role.

MRP RNA

MRP RNP was originally characterized as a site-specificribonuclease that cleaves an RNA sequence primingleading-strand DNA synthesis in mitochondria. TheRNA component of MRP is encoded by a nuclear geneand must be imported into the mitochondria in order toprocess mitochondrial RNA in vivo. However, cellularfractionation and immunolocalization have shown that avast majority of the RNase MRP is located in thenucleolus. Therefore, MRP RNA represents a uniquemember of the snoRNAs. MRP RNA has been found inmany eukaryotes includinghuman, yeast and plant cells.Inyeast (Saccharomyces cerevisiae) RNase MRP cleaves pre-

rRNA in a region upstream of the 5.8S rRNA and thiscleavage can be reproduced in vitro by the highly purifiedenzyme. At least three protein components have beenfound in yeast in MRP particles. Both the RNA andprotein components of MRP are essential for viability inyeast.

RNase P RNA

RNase P is an RNP responsible for the generation of themature 5’ end of tRNAs from precursor tRNAs by a singleendonucleolytic cleavage. Bacterial RNase P is composed

of a catalytic RNA subunit of 350–450 nucleotides and asmall protein subunit of about 120 amino acids. Under invitro reaction conditions of high ionic strength, the RNAitself can cleave precursors of tRNA in the absence of theprotein subunit. This is the first true RNA enzymecharacterized. However, the protein subunit is essentialfor activity in vivo. Bacterial RNase P can cleave not onlyall the different precursors of tRNAs but also other non-tRNA substrates including precursor 4.5S RNA and pre-rRNA. Eukaryotic RNase P enzymes are more complexcompared with the bacterial ones in that eukaryoticenzymes have a significantly higher protein content andattempts to show that RNA alone has catalytic activity

have not been successful.Several lines of evidence suggest that RNase P and MRP

are related to each other. Both are endoribonucleaseswhich cleave RNA to generate 5’-phosphate and 3’-hydroxyl termini in a divalent cation-dependent manner.Both have activity in the nucleus and mitochondria. BothRNase P RNA and MRP RNA are synthesized by RNApolymeraseIII and are not capped.Although their primarysequences are not highly homologous, they do containdistinct conserved regions. In addition, the two RNAs can

be folded into strikingly similar secondary structures.BotRNase P and MRP share some common protein components. Moreover, both of the in vitro substrates recognizeby RNase MRP, the mitochondrial D loop region and thyeast pre-rRNA, can also be cleaved in vitro by RNase PThe structural and functional similarities between RNasMRP and RNase P have led to the suggestion that RNase

RNA and MRP RNA originated from a commoancestor.

The 7SL RNA Component of the SignalRecognition Particle

The targeting and insertion of proteins into the membranof the rough endoplasmic reticulum (ER) of eukaryoticells is an essential step in the biosynthesis of both secreteand membrane proteins. This process is mediated by cytoplasmic RNP complex called signal recognitioparticle (SRP). Components of SRP and the SRP receptohave been found in a large variety of organisms. Imammals, SRP consists of six proteins of 9, 14, 19, 54, 6and 72 kDa, and a single RNA molecule, called 7SL RNAwhich probably plays a scaffolding role for the precisassembly of SRPparticles. 7SLRNA is a molecule of abou300 nucleotides and is highly conserved in evolution. It itranscribed by RNA polymerase III and is not capped.Th7SL RNA contains regions of homology to Alu sequencewhich is an extremely abundant, repetitive sequence ihuman genome. It is believed that 7SL RNA is thprogenitor of Alu elements. The RNA component of Scerevisiae SRP, scR1, bears little sequence homology t7SL, but has a very similar secondary structure and is thfunctional counterpart of mammalian 7SL RNA.

4.5S RNA

Transport of protein across bacterial cytoplasmic membrane is evolutionarily related to transport of protein

across the ER membrane in eukaryotic cells. In E. coli , thtranslocation into the periplasm of secretory proteinmostly depends on the so-called ‘general secretory pathway’. However, an alternative SRP-dependent targetinpathway has also been identified. E. coli SRP is relativelsimple and contains a 4.5S RNA and a single protein whicis homologous to mammalian SRP54. 4.5S RNA (11nucleotides) is a stable, abundant RNA that is essential foviability. It forms an extended stem–loop structure, whicis homologous to the most conserved domain of 7SL RNA




Alu transcripts

The Alu family is an extremely abundant, repetitivesequence representing around 6–13% of human genomicDNA. They were named after the AluI restriction sitewithin this consensus sequence. Consensus Alu sequencesare approximately 300 bp in length, and consist of two

similar, but distinct monomers linked by an oligo-d(A)tract. At the designated 3’ end of the Alu transcript there isan oligo-d(A) of variable length.

Despite the high numbers of Alu repeats in the humangenome, Alu RNA transcripts are very scarce in normalcultured cells. There are two main forms of pol III-transcribed cytoplasmic Alu transcripts. Full-length AluRNA (flAlu) contains the typical dimeric Alu sequencesand the 3’ poly(A). A fraction of flAlu is processed intomore stable, small cytoplasmic Alu (scAlu) RNA, whichcorresponds to the left monomer of the dimeric structure.The functional role of Alu transcripts remains a mystery.However, cell stress, viral infection and translational

inhibition increase the abundance of human Alu RNAs,suggesting a physiological role of Alu RNAs.

Telomerase RNAs

Telomeres are the specialized structures comprising thetermini of eukaryotic chromosomes. In nearly all eukar-yotes examined, the telomeric DNA consists of tracts of tandemly repeated sequences extending to the chromoso-mal ends. These telomeric repeats are generally short, G-rich tandem repeats and are required for chromosomal

stability and complete replication.Cellular DNA polymerase can only synthesize in the 5’

to 3’ direction, and requires an RNA primer to initiatesynthesis. Without some form of terminal replication,chromosomes would progressively recede from their endssince the initiating RNA primers have to be removed. It istelomerase, a unique cellular reverse transcriptase, thatplays a critical role in telomere maintenance.

Telomerase is an RNP particle which contains an RNAtemplate as an integral part of the enzyme. The RNAcomponent contains a sequence complementary to thetelomeric repeats and serves as the template for theirsynthesis. The gene encoding telomerase RNA has been

sequenced from more than 20 ciliate species, yeast, mouseand human. The primary sequences of telomerase RNAshave diverged considerably, however, the secondarystructure of telomerase RNA is highly conserved. Telo-merase RNA is transcribed by RNA polymerase III inciliates, however, in yeast and mammals, it is transcribedby RNA polymerase II. In yeast, a portion of telomeraseRNA is polyadenylated, although the functional signifi-cance of this is unknown.

RNA Primers for Okazaki Fragments

During cellular DNA replication, the leading strand isynthesized continuously in the direction of replicatiofork propagation. An antiparallel template is used fosynthesis of the lagging strand, which therefore must bmade as a series of discontinuous segments of 100–20

nucleotides called Okazaki fragments. No known DNApolymerase can initiate the gene duplication de novoinstead, the enzyme can only extend replication in a 5’ to 3direction from the free hydroxyl terminus of a preexistinoligonucleotide base paired with the template. In both thprokaryotes and eukaryotes, it is the short RNA primerthat provide the free 3’-hydroxyl terminus. These RNAprimers normally are 8–12 nucleotides long. In prokaryotes, they are synthesized by a special enzyme, DNAprimase. In eukaryotes, the priming activity exists as subunit of DNA polymerasea. On the leading strand, onla single RNA primer is needed. On the lagging strandhowever, a new initiator RNA is needed to prime eac

Okazaki fragment. Eventually, all of the RNA primermust be degraded, and the DNA segments extended an joined for replication to be completed.

Non-protein-coding mRNAs

Ever since the discovery of the split gene in eukaryoteintrons have been considered as ‘junk DNA’, while spliceexons are meaningful sequences which code for proteinsThe first blow to this traditional view came several yearago when most mammalian snoRNAs were found to b

processed from introns of pre-mRNA. The most stunninchallenge to the traditional definition of exon and intronthe elucidation of UHG (U22 host gene) structure. UHencodes eight box C/D snoRNAs (U22, U25–U31) withiits nine introns. The spliced UHG mRNA is poorlconserved between human and mouse, lacks a long opereading frame, and is rapidly degraded in the cytoplasmThe introns, on the other hand, are highly homologous. Iseems that only the introns of UHG encode functionwhereas the exons are ‘junk DNA’ to be discarded. Severasimilar genes have been identified, including Gas5, whicencodes several snoRNAs, and U19H (U19 host gene).

There are several examples of non-protein-codin

mRNAs which have been implicated in important regulatory functions. For example, Xist RNA is essential foinactivation of most genes along the X-chromosome ifemale mammals and Drosophila; Xlsirt RNAs are a cruciapart of a genetic pathway necessary for the normal patterformation in Xenopus; Pgc RNA is required for germlindevelopment in Drosophila; etc.


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Maternal mRNAs in Development

Early development in many animal species is programmedby maternal mRNAs inherited by the fertilized egg. Manyof these maternal mRNAs are translationally dormant inimmature oocytes, but are translated selectively in atemporally specific manner during oocyte maturation,

fertilization or early embryogenesis. The protein productsfrom these maternal mRNAs are known to play crucialroles in developmental events including oocyte matura-tion, cell cycle progression and determination of bodypattern.

Further Reading

Baserga SJ and Steitz JA (1993) The diverse world of small

ribonucleoproteins. In: Gesteland RF and Atkins JF (eds)The RNA

World , pp. 359–382. Cold Spring Harbor, NY: Cold Spring Harb

Laboratory Press.

Grosjean H and Benne R (1998) Modification and Editing of RNA

Washington, DC: ASM Press.

Sharp PA (1994) Split genes and RNA splicing.Cell 77: 805–815.

Simons RW and Grunberg-Manago M (1998) RNA Structure an

Function. Cold Spring Harbor, NY: Cold Spring Harbor Laborato

Press.

Soll D and RajBhandary UL (1995) tRNA: Structure, Biosynthesis, anFunction. Washington, DC: ASM Press.

Staley JP and Guthrie C (1998) Mechanical devices of the spliceosom

motors, clocks, springs and things. Cell 92: 315–326.

TollerveyD andKiss T (1997)Function andsynthesis of small nucleol

RNAs. Current Opinion in Cell Biology 9: 337–342.

Walter P and Johnson AE (1994) Signal sequence recognition an

protein targeting to the endoplasmic reticulum membrane. Annu

Review of Cell Biology 10: 87–119.

Zimmermann RA and Dahlberg AE (1996) Ribosomal RNA Structur

Evolution, Processing, and Function in Protein Biosynthesis. Boc

Raton, FL: CRC Press.


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