Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 195

The ribosome is a very busy place during translation, with numerous factors and tRNAs cycling quickly through the A and P sites. Different factors have to enter the ribosome for each of the steps and then leave when they have finished their functions. One way the complexity of the system seems to be reduced is by having the various factors and tRNAs mimic each other′s structure, which allows them to bind to the same sites on the ribosome. For example, the translation factor EF-G seems to be roughly the same shape as the translation factor EF-Tu bound to an aa-tRNA. This may allow EF-G to enter the A site, displace the tRNA (now attached to the growing polypeptide), and move it to the P site. Another example is the mimicry between the tRNAs and the release factors. The release factors resemble tRNAs in shape, but they seem to bind to specific terminator codons through interactions between amino acids in the release factors and nucleotide bases in the termination codon, rather than through base pairing between the codon and the anticodon on a tRNA. When the peptidyltransferase attempts to transfer the polypeptide to the release factor in the A site, it sets in motion the string of events that cause translation to be terminated and the polypeptide and mRNA to be released from the ribosome. It is an attractive idea that the release factors replaced what were once terminator tRNAs that responded to these terminator codons. Perhaps, in the earliest forms of life, everything in translation was done by RNA; now, RNA is used to make proteins, and the proteins, being more versatile, play many of the roles previously played by RNA.