Regulation of proline dehydrogenase activity in Escherichia coli by leucyl-, phenylalanyl-tRNA:protein transferase.

We have investigated the basis for an increased level of proline dehydrogenase activity, the most prominent phenotype displayed by a mutant of Escherichia coli lacking leucyl-, phenylalanyl-tRNA:protein transferase. A dehydrogenase preparation of approximately 95% purity from the transferaseless mutant contained glycine, which is not an acceptor determinant in the transfer reaction, as the sole NH2-terminal residue detected by the dansylation (5-dimethylaminonaphthalene-1-sulfonyl) procedure. This preparation accepted only 0.06 eq of radioactive phenylalanine under conditions of stoichiometric acylation with the purified transferase. The acylated acceptor represented a minor contaminant clearly distinguished from the dehydrogenase protein by autoradiography after gel electrophoresis in the reduced, denatured state. The specific activity of the proline dehydrogenase protein from the transferaseless mutant and from its parental strain was estimated to be about 7.5 units/mg, both by competition radioimmunoassay using crude solubilized extracts as competing antigen and by densitometer scanning of purified preparations after gel electrophoresis. These results establish that proline dehydrogenase is not an acceptor in the transfer reaction and that the elevated activity in the transferaseless strain is due to an increase in the number of enzyme molecules rather than in their individual catalytic activity.