ROLES OF LYSP AND CADC IN MEDIATING THE LYSINE REQUIREMENT FOR ACID INDUCTION OF THE ESCHERICHIA-COLI CAD OPERON

Expression of the Escherichia coil cadBA operon, encoding functions re quired for the conversion of lysine to cadaverine and for cadaverine e xcretion, requires at least two extracellular signals: low pH and a hi gh concentration of lysine. To better understand the nature of the lys ine-dependent signal, mutants were isolated which expressed a cadA-lac Z transcription fusion-in the absence of lysine while retaining pH reg ulation. The responsible mutation in one of these isolates (EP310) was in cadC, a gene encoding a function necessary for transcriptional act ivation of cadBA. This mutation (cadC310) is in a part of the gene enc oding the periplasmic domain of CadC and results in an Arg-to-Cys chan ge at position 265, indicating that this part of the protein is involv ed in responding to the presence of lysine. Three other mutants had mu tations mapping in or near lysP (cadR), a gene encoding a lysine trans port protein that has previously been shown to regulate cadA expressio n. One of these mutations is an insertion in the lysP coding region. T hus, in the absence of exogenous lysine, LysP is a negative regulator of cadBA expression. Negative regulation by LysP was further demonstra ted by showing that lysP expression from a high-copy-number plasmid re ndered cadA-lacZ uninducible. Expression of cad4-lacZ in a strain carr ying the cadC310 allele, however, was not affected by the plasmid-expr essed lysP. Cadaverine was shown to inhibit expression of the cadA-lac Z fusion in cadC(+) cells but not in a cadC310 background.