Analysis of factors involved in serine-sensitivity in bacterial cells

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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