Hulanicka M D, Hallquist S G, Kredich N M, Mojica-A T
J Bacteriol. 1979 Oct;140(1):141-6. doi: 10.1128/jb.140.1.141-146.1979.
A technique based on resistance to azaserine was used to isolate mutants lacking O-acetylserine sulfhydrylase B, one of two enzymes in Salmonella typhimurium capable of synthesizing L-cysteine from O-acetyl-L-serine and sulfide. The mutant locus responsible for this defect has been designated cysM, and genetic mapping suggests that cysM is very close to and perhaps contiguous with cysA. Strains lacking either O-acetylserine sulfhydrylase B or the second sulfhydrylase, O-acetylserine sulfhydrylase A (coded for by cysK), are cysteine prototrophs, but cysK cysM double mutants were found to require cysteine for growth. O-Acetylserine sulfhydrylase B was depressed by growth on a poor sulfur source, and depression was dependent upon both a functional cysB regulatory gene product and the internal inducer of the cysteine biosynthetic pathway, O-acetyl-L-serine. Furthermore, a cysBc strain, in which other cysteine biosynthetic enzymes cannot be fully repressed by growth on L-cystine, was found to be constitutive for O-acetylserine sulfhydrylase B as well. Thus O-acetylserine sulfhydrylase B is regulated by the same factors that control the expression of O-acetylserine sulfhydrylase A and other activities of the cysteine regulon. It is not clear why S. typhimurium has two enzymes whose physiological function appears to be to catalyze the same step of L-cysteine biosynthesis.
一种基于对重氮丝氨酸抗性的技术被用于分离缺乏O - 乙酰丝氨酸巯基酶B的突变体，O - 乙酰丝氨酸巯基酶B是鼠伤寒沙门氏菌中两种能够从O - 乙酰 - L - 丝氨酸和硫化物合成L - 半胱氨酸的酶之一。导致这种缺陷的突变基因座已被命名为cysM，遗传图谱表明cysM非常接近cysA，甚至可能与cysA相邻。缺乏O - 乙酰丝氨酸巯基酶B或第二种巯基酶O - 乙酰丝氨酸巯基酶A（由cysK编码）的菌株是半胱氨酸原养型，但发现cysK cysM双突变体生长需要半胱氨酸。在劣质硫源上生长会使O - 乙酰丝氨酸巯基酶B受到抑制，这种抑制依赖于功能性的cysB调控基因产物以及半胱氨酸生物合成途径的内源诱导物O - 乙酰 - L - 丝氨酸。此外，还发现一种cysBc菌株，在该菌株中，其他半胱氨酸生物合成酶不能被L - 胱氨酸的生长完全抑制，其O - 乙酰丝氨酸巯基酶B也是组成型的。因此，O - 乙酰丝氨酸巯基酶B受控制O - 乙酰丝氨酸巯基酶A表达和半胱氨酸调节子其他活性的相同因素调控。目前尚不清楚为什么鼠伤寒沙门氏菌有两种酶，其生理功能似乎是催化L - 半胱氨酸生物合成的同一步骤。
