Abstract

This work demonstrates the existence of Escherichia coli K-12 of two L-serine deaminating enzymes, L-serine deaminase (L-SD)#1 and L-SD#2. It demonstrates that the structure of L-SD#1 is coded by the sdaA gene, which has been cloned and sequenced. The entire sdaA gene was fused to the lacZ gene by mutating the stop-codon of sdaA and ligating in-frame to lacZ. The fused gene directed the formation of a large protein showing both L-SD and $\beta$-galactosidase activities. L-SD#1 has been extensively purified for the first time by use of a three-part fusion protein and some of its characteristics studied. L-SD#2 is synthesized in wild-type cells in LB medium. A mutation in the sdaX gene established its expression in minimal medium. An insertion in sdaB abolished L-SD#2 activity in an sdaA::Cm$\sp{\rm r}$ sdaX strain, allowing the sdaB gene to be cloned by restoring growth on L-serine. The sdaA gene was located at 41 minutes; sdaB and sdaX both were located near 60.1 minutes and may be the same gene. Some experiments directed towards the identification of the metabolic role of L-SD are included.