(msioA) Present Next to the Anticodon Contributes to the Decoding Efficiency of the tRNA†

A Salmonella typhimurium LT2 mutant which harbors a mutation (miaB2508::Tn10dCm) that results in a reduction in the activities of the amber suppressors supF30 (tRNACUA Tyr ), supD10 (tRNACUA Ser ), and supJ60 (tRNACUA Leu ) was isolated. The mutant was deficient in the methylthio group (ms) of N-(4-hydroxyisopentenyl)2-methylthioadenosine (msioA), a modified nucleoside that is normally present next to the anticodon (position 37) in tRNAs that read codons that start with uridine. Consequently, the mutant had iA37 instead of msioA37 in its tRNA. Only small amounts of ioA37 was found. We suggest that the synthesis of msioA occurs in the following order: A-373iA373msiA373msioA37. The mutation miaB2508::Tn10dCm was 60% linked to the nag gene (min 15) and 40% linked to the fur gene and is located counterclockwise from both of these genes. The growth rates of the mutant in four growth media did not significantly deviate from those of a wild-type strain. The polypeptide chain elongation rate was also unaffected in the mutant. However, the miaB2508::Tn10dCm mutation rendered the cell more resistant or sensitive, compared with a wild-type cell, to several amino acid analogs, suggesting that this mutation influences the regulation of several amino acid biosynthetic operons. The efficiencies of the aforementioned amber suppressors were decreased to as low as 16%, depending on the suppressor and the codon context monitored, demonstrating that the ms group of msioA contributes to the decoding efficiency of tRNA. However, the major impact of the msio modification in the decoding process comes from the io group alone or from the combination of the ms and io groups, not from the ms group alone.