Positive control in the D-serine deaminase system of Escherichia coli K-12.

Two new types of D-serine deaminase (Dsdase)-negative mutants have been isolated and characterized. The first fails to synthesize a functional dsdC gene product as a result of dsdC- (regulator negative) mutations. The mutations lie in the dsdC region, are cis and trans recessive to dsdC+, and give rise to revertants of novel regulatory phenotype. The second class consists of Dsdase-negative lysogens in which the phenotype is the result of the integration of lambdac1857 Sam7 into the dsdC region. Lambda lysates derived from two of the Dsdase-negative lysogens can transduce the structural gene for Dsdase (dsdA) but not the dsdC region. The dsdC+ gene product had no repressor effect on constitutive synthesis in a strain containing a dsdO (initiator constitutive) and a dsdC- mutation. These and other findings indicate that control of Dsdase synthesis is strictly positive. The partial trans effect of the dsdC+ gene product on constitutive synthesis in dsdCc (regulator constitutive) strains can thus be explained by "subunit mixing" between active dsdCc subunits and dsdC+ subunits which are inactive in the absence of the inducer, D-serine. The order of genes in the dsd region is supN-dsdC-dsdP-dsdA-aroC.