ESCHERICHIA-COLI CAD OPERON FUNCTIONS AS A SUPPLIER OF CARBON-DIOXIDE

We examined the gene expression of the Escherichia coil cad operon, wh ich consisted of the genes cadB and cadA (lysine decarboxylase), using cells possessing cadB-lacZ fusion gene. The cad operon was expressed when O-2 was limited, and the expression was optimal at pH 6.3. The be ta-galactosidase activity was lowered by the addition of sodium carbon ate to the medium. The expression of the cad operon was reduced in cel ls containing the plasmid-encoding ornithine decarboxylase, which prod uced carbon dioxide, indicating that the gene expression of the cad op eron was regulated by carbon dioxide (or its derivatives). It is known that the Krebs cycle is a major pathway for producing carbon dioxide, and that its activity is repressed when O-2 is limited. Thus, our pre sent results suggested that the physiological role of the cad operon i s to supply carbon dioxide when its internal level is lowered under O- 2-limiting conditions at a low pH.