PLEIOTROPIC EFFECTS INDUCED BY MODIFICATION DEFICIENCY NEXT TO THE ANTICODON OF TRANSFER-RNA FROM SALMONELLA-TYPHIMURIUM-LT2

A strain of Salmonella typhimurium LT2 was isolated, which harbors a mutation acting as an antisuppressor toward an amber suppressor derivative, supF30, of tRNA1Tyr. The mutant is deficient in cis-2-methylthioribosylzeatin[N6-(4-hydroxyisopentenyl)-2-methylthioadenosine, ms2106A], which is a modification normally present next to the anticodon (position 37) in tRNA reading codons starting with uridine. The gene miaA, defective in the mutant, is located close to and counterclockwise of the purA gene at 96 min on the chromosomal map of S. typhimurium with the gene order mutL miaA purA. Growth rate of the mutant was reduced 20 to 50%, and the effect was more pronounced in media supporting fast growth. Translational chain elongation rate at 37.degree. C was reduced from 16 amino acids per s in the wild-type cell to 11 animals acids per s in the miaA1 mutant in the four different growth media tested. The cellular yield in limiting glucose, glycerol, or succinate medium was reduced for the miaA1 mutant compared with wild-type cells, with 49, 41, and 57% reductions, respectively. The miaA1 mutation renders the cell more sensitive or resistant toward several amino acid analogs, suggesting that the deficiency in ms2106A influences the regulation of several amino acid biosynthetic operons. We suggest that tRNAPhe, lacking ms2106A, translates a UUU codon in the early histidine leader sequence with lowered efficiency, leading to repression of the his operon.