cell division that is much like theblebbing of PMN. Goldfarb notes that,although homologous genes have notbeen identified, ‘it doesn’t mean anautologous process does not occur’ inmammalian cells. A similar blebbingevent has been observed in Bloom’sdisease – a genetic disease that resultsfrom a mutation in a DNA helicasegene – in which ‘nuclear microvesicles’are released into the cytoplasm [6].Regardless, Klionsky finds the study ofbiodegradation ‘fascinating overall’.Although most research has focused on

biogenesis, he says ‘there is always thishomeostasis. You have to have both[processes], you can’t have just one’.

Although the research undertakenhere might not lead to thedevelopment of novel therapeuticagents, it brings us a step closer tounderstanding the ways in whicheukaryotic cells recycle materials inspecific and refined ways.

References1 Roberts, P. et al. (2003) Piecemeal

Microautophagy of Nucleus in Saccharomycescerevisiae. Mol. Biol. Cell 14, 129–141

2 Klionsky, D.J. and Ohsumi, Y. (1999)Vacuolar import of proteins and organellesfrom the cytoplasm. Annu. Rev. Cell. Dev. Biol.15, 1–32

3 Reggiori, F. and Klionsky, D.J. (2002)Autophagy in the eukaryotic cell. Euk. Cell 1,11–21

4 Scott, S.V. et al. (2001) Cvt19 is a receptor forthe cytoplasm-to-vacuole targeting pathway.Mol. Cell 7, 1131–1141

5 Talloczy, Z. et al. (2002) Related regulation ofstarvation- and virus-induced autophagy bythe eIF2alpha kinase signaling pathway.Proc. Natl. Acad. Sci. U. S. A. 99, 190–195

6 van Brabant, A.J. et al. (2000) DNA helicases,genomic instability and human geneticdisease. Annu. Rev. Genomics Hum. Genet. 1,409–459

DDT Vol. 8, No. 6 March 2003 updatenews

1359-6446/03/$ – see front matter ©2003 Elsevier Science Ltd. All rights reserved. PII: S1359-6446(03)02635-7 239www.drugdiscoverytoday.com

Scientists expand the genetic codeVida Foubister, freelance writer

Ever wonder whether mankind wouldbe better off with a genetic code thatuses 21 amino acids, one more than the standard 20, to make proteins? It soon might be possible to find out. Scientists at The Scripps ResearchInstitute (http://www.scripps.edu/) have created a bacterium thatsynthesizes an unnatural amino acid,p-aminophenylalanine (pAF), andincorporates it into proteins with afidelity and efficiency rivaling that ofthe 20 natural amino acids [1].

‘They’ve really redesigned Escherichiacoli,’ said Michael Ibba, AssistantProfessor of Microbiology at The OhioState University (http://www.osu.edu/).‘They chose what amino acid to add tothe genetic code of this organism andthey gave the organism the machineryto make the amino acid and use it.’

Although there is nothing specialabout pAF, a known synthetic aminoacid with a structure similar to tyrosine,this study demonstrates the potential tomake changes that are more

interesting, both scientifically andtherapeutically. ‘The nice thing abouttheir approach is that it’s modular,’ said Virginia Cornish, Assistant Professorof Chemistry at Columbia University(http://www.columbia.edu/). ‘Whilethey can only have 21 total aminoacids, what that 21st amino acid isthey can vary fairly easily.’

A complete systemThe challenge facing the Scrippsscientists was to put all the piecesnecessary for the biosynthesis of pAFand its incorporation into protein inE. coli and ensure they worked together.‘In order to go from 20 to 21 aminoacids, we had to add a biosyntheticpathway, add a synthetase, add a tRNAand add a codon for that new aminoacid,’ said Ryan A. Mehl, now anAssistant Professor of Chemistry atFranklin & Marshall College(http://www.fandm.edu/). ‘We stolethe machinery from other organismsand modified it.’

First, a biosynthetic pathway thatenabled E. coli to make pAF from simplecarbon sources was needed. Threegenes from Streptomyces venezuelae,which lead to the production ofp-aminophenylpyruvic acid as anintermediate metabolite in theconversion of chorismate tochloramphenicol, were introduced intoE. coli (Fig. 1). A native E. coli enzyme,aromatic aminotransferase, completesthe biosynthesis to pAF.

To incorporate the resulting 21st aminoacid into protein, a tRNA–synthetase pairfrom Methanococcus jannaschii wasintroduced into the same bacterium.The tRNA was chosen because of itsspecificity for TAG, a nonsense codonthat normally tells the cell machinery tostop making protein. However, in thissystem, the synthetase was altered torecognize pAF and load it onto thetRNA. Because of that modification, theunnatural amino acid is incorporatedinto protein in response to the TAGstop codon.

To determine whether pAF was beinggenerated, amino acids were extractedfrom cells grown in minimal media,separated by HPLC and then measuredby MS. Finally, the fourth codon of thesperm whale myoglobin gene wasconverted to a TAG stop codon, andthe insertion of pAF into the protein atthis site was confirmed by N-terminalsequencing.

Exploiting this new chemistryThe ability to site-specifically modifyproteins holds tremendous potentialboth for basic scientific research anddrug design. ‘Historically, the way we’vetried to understand biological systems isby isolating components in test tube

and analyzing them there,’ Cornishsaid. ‘But what you really want to knowis how they function in the cell.’

The possibilities include using abiophysical probe, such as fluorophores,to directly tag a protein within livingcells. ‘This approach would allow you tofollow the protein in real time with amicroscope and watch various biologicalprocesses,’ said Christopher Anderson,Graduate Student at Scripps. Anotherprobe called a photocrosslinker, whichcan be activated by ultraviolet light tocrosslink proteins, could be used tostabilize transient protein–proteininteractions within cells [2,3].

There are also several potentialtherapeutic protein applications. Theaddition of a ketone [4], a functionalgroup that is not present in any of thenatural 20 amino acids, would givescientists a ‘reactive handle,’ Andersonsaid. ‘You create a site that is totallyunique in that molecule and can beselectively modified.’ Scientists couldalso use this approach to site specificallyattach polyethylene glycol, a longpolymer that improves bioavailability,to therapeutic proteins. Today, thePEGylation of commercially availableproteins, such as insulin anderythropoietin, is nonspecific.

It might also be possible to use theinsertion of an unnatural 21st aminoacid to enable E. coli to make post-translational modifications that are

essential for the function of sometherapeutic proteins. Erythropoietin, for example, requires glycosylation.

Future workThe Scripps lab, lead by Peter G. Schultz,Professor of Chemistry, plans to pursuemany of those applications. It is also isworking to develop new systems thatenable E. coli to incorporate unnaturalamino acids with chemical structuresthat differ from pAF. As well asmodifying tRNA–synthetase pairs, thegroup will work to add amino acidbiosynthetic pathways because of theirpotential to reduce costs by eliminatingthe need for unnatural amino acids inthe growth media. In the future, thesebiosynthetic pathways might alsoenable applications in more complexorganisms where feeding the animallarge amounts of an unnatural aminoacid could be toxic.

References1 Mehl, R.A. et al. (2003) Generation of a

bacterium with a 21 amino acid genetic code.J. Am. Chem. Soc. 125, 935–939

2 Chin, J.W. and Schultz, P.G. (2002) In vivophotocrosslinking with unnatural amino acidmutagenesis. ChemBioChem. 3, 1135–1137

3 Chin, J.W. et al. (2002) Addition of aphotocrosslinking amino acid to the geneticcode of Escherichia coli. Proc. Nat. Acad. Sci. U. S. A. 99, 11020–11024

4 Wang, L. et al. (2003). Addition of the ketofunctional group to the genetic code ofEscherichia coli. Proc. Nat. Acad. Sci. U. S. A.100, 56–61

update news DDT Vol. 8, No. 6 March 2003

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Figure 1. A novel bacterium with a21 amino acid genetic code converts(a) chorismate, an intermediate in thesynthesis of aromatic amino acids, into(b) p-aminophenylalanine (pAF) andincorporates this unnatural amino acidinto proteins in response to the ambernonsense codon UAG. Figure reproduced,with permission, from Ref. [1].

HO H H

O

O OH

OH

O

NH2

O

OH

NH2

(a) (b)

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