Isolation and characterization of cAMP suppressor mutants of Escherichia coli K12

We have isolated spontaneous and chemically induced revertants of cya mutant strains of Escherichia coli. Three different classes of revertants were obtained. One class consisted of primary site revertants; a second class was pseudorevertants that had phenotypically reverted to wild type but retaining the original cya mutant and the third class of revertants, designated csm, were pseudorevertants hypersensitive to exogenous cAMP. Transductional analysis of the csm mutation indicated the mechanism of suppression in these strains was intergenic. The csm mutation and hypersensitivity to cAMP map in or near the crp gene. Growth of the csm strains on PTS (phosphoenolpyruvate phosphotransferase system) and non-PTS substrates was inhibited by 5 mM cAMP. The csm strains were found to accumulate toxic levels of methylglyoxal when grown on non-PTS substrates in the presence of exogenous cAMP. All csm strains were sensitive to catabolite repression mediated by alpha-methylglucoside. Revertants selected as resistant to cAMP fell into four major classes that could be distinguished by their fermentation patterns in the presence and absence of cAMP as well as by their growth response to streptomycin in the presence of cAMP.