AmpG, a signal transducer in chromosomal β‐lactamase induction

Abstract

The chromosomal ampC beta-lactamase in Citrobacter freundii and Enterobacter cloacae is inducible by beta-lactam antibiotics. When an inducible ampC gene is introduced on a plasmid into Escherichia coli together with its transcriptional regulator ampR, the plasmid-borne beta-lactamase is still inducible. We have isolated mutants, containing alterations in a novel E. coli gene, ampG, in which a cloned C. freundii ampC gene is unable to respond to beta-lactam inducers. The ampG gene was cloned, sequenced and mapped to minute 9.6 on the E. coli chromosome. The deduced amino acid sequence predicted AmpG to be a 53 kDa, transmembrane protein, which we propose acts as a signal transducer or permease in the beta-lactamase induction system. Immediately upstream of ampG there is another 579-base-pair-long open reading frame (ORF) encoding a putative lipoprotein shown to be non-essential for beta-lactamase induction. We have found that ampG and this ORF form an operon, whose promoter is located in front of the ORF. Located closely upstream of the putative promoter is the morphogene bolA, which is transcribed in the opposite orientation. However, using transcription fusions, we have found that the ampG transcription is not regulated by bolA. In addition, we show that transcription is probably not regulated by either the starvation specific sigma factor RpoS, which controls bolA, or by AmpD the negative regulator for ampC transcription.

A Common System Controls the Induction of Very Different Genes

“…Assuming a mechanism of induction in P. vulgaris similar to that proposed for AmpC P-lactamase (Lindquist et al, 1993), loss of inducibility could be caused either by a mutational defect in cumR, or in the postulated, ampG-analogous signal-transducer gene, tentatively named cumG. In agreement with this expectation, two types of mutants were distinguished when inducible expression of heterologous AmpC P-lactamase was tested after transconjugation of pMDlOl(ampR+, ampC').…”

“…Plasmid pGKS273-3 (ampG') was kindly provided by Dr Gisela Korfmann, Bayer AG, Leverkusen, Germany. It contains the cloned gene ampG of E. coli JRG582 (Lindquist et al, 1993) on an EcoRV fragment inserted into the EcoRV site of pACYC184 (G. Korfmann, unpublished results). For the construction of pMD301, ampG on a 5.5-kbp EcoRI fragment from pGKS273-3 (Schmidt, 1991) was inserted into the EcoRI site of pKT231.…”

“…Detailed studies of the AmpC induction mechanism have revealed the essential participation of additional genes, ampR, ampG and the ampDE operon. These encode a transcriptional regulator (Lindberg et al, 1985;HonorC et al, 1986;Lindquist et al, 1989a), a transducer of the external induction signal (Korfmann and Sanders, 1989;Lindquist et al, 1993) and a negative modulator of AmpD (Lindberg et al, 1987) Enzymes. /I-Lactamase (EC 3.5.2.6); DNA-ligase (EC 6.5.1.1); RNdse A (EC 3.1.27.5); alkaline phosphatase (EC 3.1.3.1); DNA polymerase (EC 2.7.7.7); lysozyme (3.2.1.17); fumarate reductase (EC 1.3.99.1).…”

Functional analysis of yersiniabactin transport genes of Yersinia enterocolitica

“…The irp8 gene downstream of irp7 exhibits 99 % homology to ybtX of Y. pestis and significant homology to ampG, which encodes a muropeptide permease (AmpG) in Gram-negative bacteria (Lindquist et al, 1993). Similar to AmpG we also could not identify the typical consensus sequence EAAXXXG (EAA-loop) of inner membrane proteins from periplasmic-bindingprotein-dependent transport (PBT) systems within Irp8 (Braun et al, 1998 ;Boos & Lucht, 1996).…”

“…YbtS participates in Ybt biosynthesis. YbtX shows homology to AmpG, which is proposed to act as a signal transducer or permease in the β-lactamase induction system (Lindquist et al, 1993). The function of YbtX and its role in virulence are still unknown Gehring et al, 1998).…”