Analysis of factors involved in serine-sensitivity in bacterial cells

丝氨酸是细菌细胞和动物细胞的极好营养素。然而，当大肠杆菌细胞在乳酸盐（或其他碳源）上生长时，向培养基中添加丝氨酸会导致生长抑制。这种抑制作用是由丝氨酸抑制高丝氨酸脱氢酶I引起的，高丝氨酸脱氢酶I参与苏氨酸-异亮氨酸生物合成。我们发现大肠杆菌中有几个因素影响丝氨酸敏感性。我们已经克隆并测序了增强丝氨酸敏感性的基因。sseA和sseB是两个开放的阅读框架。sseA或sseB基因的导入，或两者同时导入，提高了大肠杆菌细胞的丝氨酸敏感性。sseA基因引起了更强的增强比sseB。推导的氨基酸序列的SseA显示出相当大的相似性与牛肝rhodanese，催化硫代硫酸盐的硫转移。我们观察到在携带sseA基因质粒的大肠杆菌细胞中罗丹酸酶活性增加了两倍。sseA在遗传图谱中的位置约为52 min，而sseA与cysM不同，后者编码O-乙酰丝氨酸巯基化酶-B，该酶催化硫从硫代硫酸盐转移到O-乙酰丝氨酸，其图谱位置也是52 min。尽管目前尚不清楚SseA或SseB如何增强丝氨酸-敏感性，但很可能SseA或SseB增加细胞内丝氨酸浓度

Serine is an excellent nutrient for bacterial cells and for animal cells. However, when cells of Escherichia coli are grown on lactate (or other carbon sources), an addition of serine to the medium causes growth inhibition. This inhibition is caused by inhibition by serine of homoserine dehydrogenase I, which is involved in threonine-isoleucine biosynthesis. We found that there are several factors in E.coli, which affect the serine-sensitivity. We have cloned and sequenced genes which enhance the serine-sensitivity. Two open reading frames were found and designated as sseA and sseB.Introduction of either sseA or sseB gene, or both, into E.coli cells enhanced the serine-sensitivity. The sseA gene elicited stronger enhancement than sseB.The deduced amino acid sequence of SseA showed considerable similarity with that of bovine liver rhodanese, which catalyzes sulfur transfer from thiosulfate. We observed a two-fold increase in rhodanese activity in E.coli cells harboring a plasmid carrying the sseA gene. The position of sseA in the genetic map is around 52 min. However, sseA is different from cysM which codes for O--acetylserine sulfhydrylase-B, an enzyme catalyzing sulfur transfer from thiosulfate to O-acetylserine, the map position of which is also 52 min. Although it is not clear yet how SseA or SseB enhances the serine--sensitivity, it is likely that SseA or SseB increases intracellular serine concentration