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The fidelity of the translation of the genetic code

100%

Sankaranarayanan R.

Acta Biochimica Polonica

2001

tom 48

nr 2

Aminoacyl-tRNA syn the tas es play a cen tral role in main tain ing ac cu racy dur ing the trans la tion of the ge netic code. To achieve this chal leng ing task they have to dis crim i- nate against amino ac ids that are very closely re lated not only in struc ture but also in chem i cal na ture. A 'dou ble-sieve' ed it ing model was pro posed in the late sev en ties to ex plain how two closely re lated amino ac ids may be dis crim i nated. How ever, a clear un der stand ing of this mech a nism re quired struc tural in for ma tion on syn the tas es that are faced with such a prob lem of amino acid dis crim i na tion. The first struc tural ba sis for the editing model came recently from the crystal structure of isoleucyl-tRNA synthetase, a class I synthetase, which has to dis crim i nate against valine. The structure showed the pres ence of two cat a lytic sites in the same en zyme, one for ac ti va tion, a coarse sieve which binds both isoleucine and valine, and an other for ed it ing, a fine sieve which binds only valine and rejects isoleucine. An other struc ture of the en zyme in com plex with tRNA showed that the tRNA is re spon si ble for the translocation of the misactivated amino-acid substrate from the catalytic site to the editing site. These studies were mainly fo cused on class I syn the tas es and the sit u a tion was not clear about how class II enzymes discriminate against similar amino acids. The recent struc tural and en zy matic stud ies on threonyl-tRNA synthetase, a class II en zyme, reveal how this chal leng ing task is achieved by us ing a unique zinc ion in the ac tive site as well as by em ploy ing a sep a rate do main for spe cific ed it ing ac tiv ity. These stud ies led us to pro pose a model which em pha sizes the mir ror sym met ri cal ap proach of the two classes of en zymes and high lights that tRNA is the key player in the evo lu tion of these class of enzymes.

Editing of plant mitochondrial transfer RNAs

86%

Fey J., Weil J. H., Tomita K., Cosset A., Dietrich A., Small I., Marechal-Drouard L.

Acta Biochimica Polonica

2001

tom 48

nr 2

Editing in plant mi to chon dria con sists in C to U changes and mainly af fects mes senger RNAs, thus pro vid ing the cor rect ge netic in for ma tion for the biosynthesis of mi to- chon drial (mt) pro teins. But ed it ing can also af fect some of the plant mt tRNAs encoded by the mt ge nome. In di cots, a C to U ed it ing event cor rects aC:A mis match into a U:A base-pair in the acceptor stem of mt tRNAPhe (GAA). In larch mitochondria, three C to U ed it ing events re store U: A base-pairs in the ac cep tor stem, D stem and anticodon stem, re spec tively, of mt tRNAHis (GUG). For both these mt tRNAs ed it ing of the pre cur sors is a pre req ui site for their pro cess ing into ma ture tRNAs. In po tato mt tRNACys (GCA), ed it ing con verts a C28:U42 mis match in the anticodon stem into a U28:U42 non-canonical base-pair, and reverse transcriptase minisequencing has shown that the mature mt tRNACys is fully edited. In the bryophyte Marchantia polymorpha this U res i due is en coded in the mt ge nome and evo lu tion ary stud ies sug gest that res to ra tion of the U28 res i due is nec es sary when it is not en coded in the gene. How ever, in vitro stud ies have shown that nei ther pro cess- ing of the pre cur sor nor amino acylation of tRNACys re quires C to U ed it ing at this po- si tion. But se quenc ing of the pu ri fied mt tRNACys has shown that Y is pres ent at po si tion 28, indicating that C to U editing is a prerequisite for the subsequent isomerization of U into Y at po si tion 28.

Ograniczanie wyników

2 Acta Biochimica Polonica

1 Cosset A.

1 Dietrich A.

1 Fey J.

1 Marechal-Drouard L.

1 Sankaranarayanan R.

1 Small I.

1 Tomita K.

1 Weil J. H.

2 2001

Wyniki wyszukiwania

The fidelity of the translation of the genetic code

Editing of plant mitochondrial transfer RNAs