IMPORTANCE OF FORMYLABILITY AND ANTICODON STEM SEQUENCE TO GIVE A TRANSFER RNA(MET) AN INITIATOR IDENTITY IN ESCHERICHIA-COLI

In bacteria, the free amino group of the methionylated initiator tRNA is specifically modified by the addition of a formyl group. The functional relevance of such a formylation for the initiation of translation is not yet precisely understood. Advantage was taken here of the availability of the fmt gene, encoding the Escherichia coli Met-tRNA(f)Met formyltransferase, to measure the influence of variations in the level of formyltransferase activity on the involvement of various mutant tRNA(f)Met and tRNA(m)Met species in either initiation or elongation in vivo. The data obtained established that formylation plays a dual role, firstly, by dictating tRNA(f)Met to engage in the initiation of translation, and secondly, by preventing the misappropriation of this tRNA by the elongation apparatus. The importance of formylation in the initiator identity of tRNA(f)Met was further shown by the demonstration that elongator tRNA(m)Met may be used in initiation and no longer in elongation, provided that it is mutated into a formylatable species and is given the three G . C base pairs characteristic of the anticodon stem of initiator tRNAs.