Unusual ribosome binding properties of mRNA encoding bacteriophage λ repressor (original) (raw)

1992, Nucleic Acids Research

The mRNA encoding represser cl of phage X is the only known E.coli message which starts directly with the initiation AUG codon. The ability of in vitro synthesized cl mRNA fragments (150 or 400 nts) to form ternary initiation complexes has been studied using the toeprint method. In the presence of tRNA Met f, these fragments are capable of forming the ternary complexes at the 5'-terminal AUG codon not only with 308 subunits but also with undissociated 70S ribosomes (70S tight couples). In the latter case, no binding at other positions of cD mRNA can be detected at all. The starting region of cl mRNA has a single stranded conformation and is highly enriched in Aresidues. This feature of cl mRNA RBS is suggested to be the main factor which allows cl mRNA to form the initiation complex with the ribosome. Unlike SOS subunits, the bonding to 70S tight couples is not affected by any of the initiation factors, although it is as efficient as that to 30S subunits supplemented with the factors. 308 subunits prefer to associate with the internal RBSs of the preformed mRNA molecules, provided that they are not sequestered by the secondary structure. On contrast, 70S tight couples tend to avoid extra sequences upstream of the codon directed to the I? site and occupy a position as close as possible to the 5'-end of the message. This has been found to be the case both for tRNA Met f and for elongator tRNA GIU 2. The structural features of mRNA RBSs which influence their different binding for SOS subunits and 70S ribosomes are discussed.