Riboswitches

Sharon Epstein30/03/2006

Frontiers in Metabolome sciencesFeinberg Graduate School

Outline Introduction Concepts Evolution Structure Mechanisms Methods Examples Applications Conclusion

Introduction

Pubmed search for “riboswitches”: Reviews: 12 Articles: 57 Pubmed search for “micro RNA”: Reviews: 167 Articles: 740

Introduction

Riboswitches were “discovered” in the beginning of the 21st century

The idea was known but could not be proved

Introduction

Concept

Riboswitches: “ method of controlling gene expression. (…)a sequence of RNA that, through its secondary and tertiary structure, selectively binds a specific metabolite.” (Templeton et al, 2005)

Concept

When a metabolite is bound the secondary and tertiary structure of the RNA changes affecting transcription and translation in prokaryotes and possibly mRNA processing in eukaryotes

Evolution RNA world, possible mode of regulation

in the absence of proteins General conservation of metabolites so

far known to be involved in binding Present and studied in prokaryotes with

differences in bacterial groups Present in prokaryotes mainly on 5’UTR

Evolution

Another hypothesis: it is more recent then RNA world but is present in different bacterial groups because of lateral transfer and repetitive re-invention

Focus prokaryote

Evolution

Major difference in eukaryotes: localization

Present in introns and 3’UTR, not well studied and not much data available

Found in Arabidopsis and rice, on different splice variants (one regulated one not)

Structure

Riboswiches are composed of two interdependent but distinct domains:

Aptamer domain (responsible for binding of ligand)

Expression plataform (responsible for impacting gene expression)

Structure

Mechanism

Three known mechanisms: Formation of intrinsic terminator

stem (inhibits transcription by inducing its termination)

Formation of complex hiding translation initiation site

Self-cleaving mechanism

Formation of intrinsic terminator stem

Inhibition of translation initiation

Self-cleaving mechanism

Methods

In line probing:

Methods

Equilibrium dialysis – radio labeled metabolite – unequal distribution

RNase H probing – DNA complementary strand – conformational change – no cleavage

Fluorescence – FMN quenched in contact with riboswitch

Examples

Coenzyme B12

One of the first to be discovered Upstream of cobalamin synthesis,

porphyrin and cobalt transport and glutamate fermentation

One of the largest aptamers with many connecting points

Coenzyme B12

Thiamine pyrophosphate

Most widespread (also found by sequence similarity in plants)

Identification of the riboswitch lead to function characterization of genes involved in the pathway

Flavin Mononucleotide

Present upstream of genes for riboflavin biosynthesis and transport pathway

Binds FMN 100 folds more tightly then riboflavin (difference: one phosphate)

Guanine and adenine

Same aptamer binds both – only one point mutation C to U (forms base-pairing with ligand)

Tertiary structure is similar – sequence only 59%

S-adenosylmethionine

S-box motif – present mainly in gram-positive bacteria

Upstream of sulfur, cysteine, SAM and methionine pathways

1:1 stoichiomestry, dependent on Mg+2

S-adenosylmethionine

Lysine

Descriminates between l and d lysine

AEC (toxic analog) is also bound by riboswitch and resistant bacteria carry mutations

Potential drug target?

Glucosamine-6-phosphate

RNA undergoes rapid self cleavage upon binding of metabolite

Mutations that affect ribozyme activity de-repress the gene

Glycine

Two aptamer upstream of glycine cleaving proteins – each binds one glycine but increases affinity of the other aptamer – increase in sensitivity

Evolutionary advantage?

Glycine

Applications

Drug target (antimicrobial)

Molecular engineering

Conclusion

Evolutionary clues Non coding regions of RNA as

undiscovered regulatory domains New role of bioinformatics Possibilities

Articles Winkler WC. (2005) Winkler WC et al. (2005) Tucker BJ et al. (2005) Templeton GW et al. (2004) Soukup JK et al. (2004) Mandal M et al. (2004) Nudler E et al. (2004) Vitreschak AG et al. (2004) Kaempfer R. (2003) Lai EC. (2003) Corbino KA et al. (2005) Altman S et al. (2005)