Altered discrimination of start codons and initiator tRNAs by mutant initiation factor 3

IF3 is essential for ensuring the fidelity of the initiation step of transl ation in bacterial cells. Mutations at residues R99 and R131 in the C-termi nal domain of the factor have previously been shown to increase initiation from the noncanonical GUA codon, Here we show that these mutant forms of IF 3 fail to discriminate against initiation from many different non-AUG codon s, They also enhance the activity of mutant tRNAs carrying changes in the t hree consecutive G-C pairs that are conserved in the anticodon stem of init iator tRNAs, In addition, the IF3 mutants stimulate initiations from leader less mRNAs and from internal initiation codons, in the absence of any SD-an ti-SD interaction. These results indicate that IF3 ensures the accuracy of initiation by inspecting both the codon-anticodon pairing and unique featur es of the initiator tRNA as well as suppressing initiation from other poten tial start sites within the mRNA.