Iron mediated methytthiobtion of tRNA as a regulator of operon expression in Escherichia coU

E. coli growing in the presence of iron-binding proteins produced tRNAP and tRNAPhe molecules containing iA instead of msiA adjacent to the anticodon. These undermodified tRNAs functioned less efficiently than the fully modified molecules when translating synthetic polynucleotides containing contiguous codons in an in vitro system, but did not limit the translation of MS2 RNA. We examined the possibility that the altered tRNAs with lowered translational efficiencies could relieve transcription termination at the trp and phe attenuators and lead to increased operon expression under iron restricted conditions. Using trpR mutants we found that there was indeed greater expression of the trp operon during iron restricted growth. This increase was attributable solely to the tRNA alteration induced by iron restriction.