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12 Ribosome Recognition of Initiator Regions in the RNA Bacteriophage Genome

Joan Argetsinger Steitz


The initiation of polypeptide chains in bacteria appears to be a highly specific and accurate process. As far as we know, both in vivo and in several well-characterized systems in vitro, ribosomes attach and start protein synthesis only at actual initiation sites (the beginnings of cistrons) on a messenger RNA. The molecular components involved in polypeptide chain initiation in E. coli have become increasingly well characterized over the past several years. In addition to the 30S ribosomal subunit and the messenger RNA containing the initiator AUG or GUG codon, formyl-N-methionyl-transfer RNAfmet, GTP and at least three proteins found in the ribosomal wash—the initiation factors IF-1, IF-2 and IF-3—are required for this process (for a review see Lucas-Lenard and Lipmann 1971). It has not been ruled out that a direct RNA-RNA interaction (involving the messenger on the one hand and ribosomal RNA or the initiator tRNA on the other) provides the basis for selectivity in ribosome binding. However, recognition of initiator regions is generally considered to be an example of a specific protein-nucleic acid interaction. One or several of the ribosomal proteins, or of the initiation factors, is presumed to select real initiation sites while discriminating against the many internal AUG and GUG codons in a natural messenger RNA. A molecular description of exactly which protein or proteins are involved and what distinguishing features of the mRNA are recognized is a first requirement towards understanding the specificity exhibited in the initiation reaction.

Polypeptide chain initiation is also a...

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