Escherichia coli formylmethionine tRNA: mutations in GGGCCC sequence conserved in anticodon stem of initiator tRNAs affect initiation of protein synthesis and conformation of anticodon loop.

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Abstract
We have generated mutants of Escherichia coli formylmethionine initiator tRNA in which one, two, and all three G X C base pairs in the GGGCCC sequence in the anticodon stem are changed to those found in E. coli elongator methionine tRNA. Overproduction of the mutant tRNAs using M13 recombinants as an expression vector and development of a one-step purification scheme allowed us to purify, characterize, and analyze the function of the mutant tRNAs. After aminoacylation and formylation, the function of mutant formylmethionyl tRNAs was analyzed in an MS2 RNA-directed in vitro protein-synthesizing system, in AUG-dependent ribosomal P site binding, and in initiation factor binding. The mutant tRNAs show progressive loss of activity in initiation, the mutant with all three G X C base pairs substituted being the least active. The mutations affect binding to the ribosomal P site. None of the mutations affects binding to initiation factor 2. We also show that there is a progressive increase in accessibility of phosphodiester bonds in the anticodon loop of the three mutants to S1 nuclease, such that the cleavage pattern of the mutant with all three G X C base-pair changes resembles that of elongator tRNAs. These results are consistent with the notion that the contiguous G X C base pairs in the anticodon stem of initiator tRNAs impart on the anticodon loop a unique conformation, which may be important in targeting the initiator tRNA to the ribosomal P site during initiation of protein synthesis.

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