Methionine Transport in Wild-type and Transport-defective Mutants of Salmonella typhimurium

Abstract

-2 p~) .Competition studies have shown that the permease has little or no affinity for the other L-amino acids commonly found in proteins. Methionine uptake was competitively inhibited by the growth inhj bitory analogues DLethionine, a-methyl-DL-methionine and DL-methionhe-DL-sulphoximine. Mutants resistant to a-methyl-methionine and methionine sulphoximine have been isolated which were severely defective in the niethionine specific permease. Two of these mutants, metP760 and rnetP761, mapped away from all previously located methionine structural and regulatory genes.

Methionine Transport in Salmonella typhimurium: Evidence for at Least One Low-affinity Transport System

“…A smaller degree of inhibition was also seen at 0.1 and 1 pM-leucine. Since the results for the wild-type strain were almost identical The final concentration of bacteria was 1OOpg dry wt ml-l. Uptake was determined from duplicate 1.0 ml samples taken at 30 s. with those for the metP mutant, the metP transport system does not transport leucine, in agreement with the results of Table 2 and Ayling & Bridgeland (1972).…”

“…MetP mutants in S. typhimurium were isolated as resistant to either of two growth inhibitory analogues, a-methylmethionine or methionine sulphoximine (Ayling & Bridgeland, 1972). Later studies showed that those methionine sulphoximine-resistant mutants which were defective in methionine transport were also defective in glutamine transport.…”

“…The isolation of metP760 and metP761 (Ayling & Bridgeland, 1972) and metP767 (Betteridge & Ayling, 1975) have already been described. The new metP mutants were isolated in three different ways :…”

“…A high-affinity transport system for L-methionine has been described in Salmonella typhimurium (Ayling & Bridgeland, 1972) and Escherichia coli (Kadner, 1974 ;Kadner & Watson, 1974). Mutants lacking the activity of this system are designated metP in S. typhimurium and metD in E. coli.…”

“…The method normally used was as described by Ayling & Bridgeland (1972). Exponentially growing cultures (usually 75 ml at an A650 of 0.6 to 0.8, determined in a Unicam SP600 spectrophotometer) were incubated with chloramphenicol (200 pg ml-l) for 30 min, washed once and resuspended in MM+ chloramphenicol (chloramphenicol medium, CM).…”