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Recent advances in ribosome crystallography revealed an atomic resolution structure of the peptidyl-transferase active site. Similarly big progress in biochemical studies of spliceosomes provided a good basis to modify our view concerning functions of these particles. In this review the problem if the ribosomes and the spliceosomes are the ribozymes is discussed.
Recently several new evidences have appeared on biological role of native short peptides. This is an overview on two of them occurring in plants: systemin and ENOD 40.
Methionyl-tRNA synthetase (MetRS) be longs to the fam ily of 20 en zymes es sen tial for pro tein biosynthesis. It links co va lently methionine with its cog nate tRNA. Crys tal structures solved for bac te rial MetRSs have given a num ber of in ter est ingin sights into en zyme ar chi tec ture and methionylation ca tal y sis. A com par i son of se quences of MetRSs be long ing to all king doms of life, as well as nu mer ous bio chem i cal and ge­netic stud ies have re vealed the pres ence of var i ous ad di tional do mains ap pended to the cat a lytic core of synthetase. They are re spon si ble for in ter ac tions with tRNA and pro teins. Ter tiary struc ture of C-terminal tRNA-binding ap pen di ces can be de duced from those de ter mined for their homo logues: tRNA bind ing pro tein 111 and en do the- lial monocyte-activating polypeptide II. Con tacts be tween MetRS and other pro teins could be me di ated not only by noncatalytic pep tides but also by struc tural el e ments pres ent in the cat a lytic core, e.g. Arg-Gly-Asp (RGD) mo tifs. Ad di tional ac tiv i ties in­volve MetRS in the main te nance of translational fi del ity and in co or di na tion of ri bo- some biogenesis with protein synthesis.
The genetic code discovered 40 years ago, consists of 64 triplets (codons) of nucleotides. The genetic code is almost universal. The same codons are assigned to the same amino acids and to the same START and STOP signals in the vast majority of genes in animals, plants, and microorganisms. Each codon encodes for one of the 20 amino acids used in the synthesis of proteins. That produces some redundancy in the code and most of the amino acids being encoded by more than one codon. The two cases have been found where selenocysteine or pyrrolysine, that are not one of the standard 20 is inserted by a tRNA into the growing polypeptide.
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