Analysis of factors involved in serine-sensitivity in bacterial cells

丝氨酸是细菌细胞和动物细胞的一种极好的营养物质。然而，当大肠杆菌细胞在乳酸(或其他碳源)上生长时，在培养基中添加丝氨酸会导致生长抑制。这种抑制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-异亮氨酸的生物合成。我们发现，在大肠杆菌中，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。发现了两个开放阅读框，分别命名为SSEA和sseB。将SSEA或SSEB基因或同时导入SSEB基因可提高丝氨酸的敏感性。SSEA基因比sseB基因具有更强的增强作用，其推导的氨基酸序列与催化硫代硫酸盐硫转移的牛肝罗丹尼斯的氨基酸序列有很大的相似性。我们观察到，在含有SSEA基因的质粒的大肠杆菌细胞中，罗丹斯酶的活性增加了两倍。SSEA在遗传图谱中的位置在52min左右。然而，SSEA不同于编码O-乙酰丝氨酸巯基酶-B的CysM，后者是一种催化硫代硫酸盐向O-乙酰丝氨酸转移的酶，其MAP位置也是52min。尽管目前尚不清楚SSEA或SSEB如何增强丝氨酸敏感性，但SSEA或SSEB很可能会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