Escherichia coli JRG582 is an ampD ampE deletion derivative of strain HfrH and accordingly it is derepressed for expression of the cloned inducible beta-lactamase gene of Citrobacter freundii, carried on plasmid pNU305. Following chemical mutagenesis of JRG582(pNU305) a cefotaxime sensitive mutant was isolated, CS51(pNU305), which produced low levels of beta-lactamase due to a mutation in the host chromosome. Two recombinant plasmids containing genomic DNA from E. coli HfrH, namely pUB5608 and pUB5611, were isolated as a consequence of their ability to restore the beta-lactam resistant phenotype to CS51(pNU305). This ability was due to direct transcriptional activation of the beta-lactamase gene, ampC, rather than complementation of the CS51 mutation. Transposon mutagenesis and subcloning showed that restoration of ampicillin resistance to CS51(pNU305) was the function of a single gene, which maps at 60.3 min on the E. coli chromosome. The gene encodes a 33 kDa protein with significant homology to members of the LysR family of bacterial activator proteins, in particular the AmpR protein from C. freundii. Homology is especially strong over the N-terminal region which includes the helix-turn-helix DNA-binding motif. This gene was shown to complement the gcvA1 mutation at 60.3 min on the E. coli chromosome, and the DNA sequence agrees exactly with the published sequence of gcvA which encodes the transcriptional activator of the inducible glycine cleavage enzyme system. It is suggested that GcvA can activate transcription of ampC by binding to the AmpR binding region upstream of ampC so as to mimic the activated state of AmpR and hence provides an example of cross-talk between DNA-binding proteins of different inducible enzyme systems.