Binding of the Citrobacter freundii AmpR regulator to a single DNA site provides both autoregulation and activation of the inducible ampC beta-lactamase gene.
Citrobacter freundii encodes an inducible chromosomal beta-lactamase. Induction requires the product of the ampR gene, which is transcribed in the opposite orientation from the ampC beta-lactamase gene. We show here that the AmpR protein acts as a transcriptional activator by binding to a DNA region immediately upstream of the ampC promoter. The DNase I footprint pattern was not affected by growth in the presence of beta-lactam inducer or by the use of extracts prepared from cells carrying the ampD2 allele leading to semiconstitutive production of beta-lactamase. It is suggested that activation of AmpR facilitates binding or open complex formation for RNA polymerase at the ampC promoter. The AmpR-binding site overlaps the ampR promoter, and beta-galactosidase activity was decreased from an ampR-lacZ transcriptional fusion when AmpR was expressed from a coresident plasmid, suggesting that ampR is autogenously controlled. The AmpR protein belongs to a family of highly homologous transcriptional activators that includes LysR, which regulates the E. coli lysine synthetase gene, and the NodD protein, which regulates expression of a number of genes involved in nodulation in Rhizobium. The lack of sequence homology to any known beta-lactam-binding protein suggests that AmpR does not bind directly to the beta-lactam inducer but interacts with a second messenger of unknown nature.
Lindquist
S
S
Department of Microbiology, University of Umeå, Sweden.
Lindberg
F
F
Normark
S
S
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics* / Q000302:isolation & purification; D001483:Base Sequence; D002954:Citrobacter / Q000235:genetics* / Q000502:physiology; D004268:DNA-Binding Proteins / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*; D005786:Gene Expression Regulation; D005796:Genes; D005798:Genes, Bacterial; D005809:Genes, Regulator; D008969:Molecular Sequence Data; D014158:Transcription, Genetic; D001618:beta-Lactamases / Q000235:genetics* / Q000302:isolation & purification
D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D001618:beta-Lactamases