AN ANTICODON SEQUENCE MUTANT OF ESCHERICHIA-COLI INITIATOR TRANSFER-RNA - POSSIBLE IMPORTANCE OF A NEWLY ACQUIRED BASE MODIFICATION NEXT TOTHE ANTICONDON ON ITS ACTIVITY IN INITIATION

Initiator tRNAs from eubacteria and chloroplasts lack a base modificat ion next to the anticodon. This is in contrast to virtually all other tRNAs from these sources. We show that a mutant Escherichia coli initi ator tRNA which has an anticodon sequence change from CAU to CUA now h as a 2-methylthio-N-6-(Delta(2)-isopentenyl)adenosine (ms(2)i(6)A) mod ification, 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 st rain of E. coli defective in biosynthesis of ms(2)i(6)A, the mutant in itiator tRNA is 10- to 12-fold less active in initiation. The mutant t RNA is aminoacylated and formylated normally in the miaA strain. Thus, the absence of the base modification affects the activity of the muta nt tRNA at a step subsequent to its formylation.