An anticodon sequence mutant of Escherichia coli initiator tRNA: possible importance of a newly acquired base modification next to the anticodon on its activity in initiation.

Initiator tRNAs from eubacteria and chloroplasts lack a base modification next to the anticodon. This is in contrast to virtually all other tRNAs from these sources. We show that a mutant Escherichia coli initiator tRNA which has an anticodon sequence change from CAU to CUA now has a 2-methylthio-N6-(delta 2-isopentenyl)adenosine (ms2i6A) modification, produced by posttranscriptional modification of A, next to the anticodon. This newly acquired base modification may be important for the function of the mutant tRNA in initiation. In a miaA mutant strain of E. coli defective in biosynthesis of ms2i6A, the mutant initiator tRNA is 10- to 12-fold less active in initiation. The mutant tRNA is aminoacylated and formylated normally in the miaA strain. Thus, the absence of the base modification affects the activity of the mutant tRNA at a step subsequent to its formylation.