The+acrAB+Efflux+Pump

The acrAB genes of E. coli encode the AcrAB multidrug efflux pump. The AcrAB pump constitutes a major drug efflux system with a broad subtrate range that confers intrinsic drug resistance (Hirakawa, 2008). The transcription of the acrAB genes is up-regulated under general stress conditions (4% ethanol,0.5M NaCl and the onset of stationary phase in LB medium). It therefore follows that mutations in these genes have been shown to render E. coli cells hyper-susceptible to growth inhibitors such as basic dyes, detergents and many antibiotics (Ma et al, 1996).

AcrAB pumps are distinguished by their ability to handle a very wide range of substrate compounds and to pump out agents, such as beta-lactam antibiotics, which cannot enter the cytoplasm but remain in the periplasm (Nikaido, 1998).

__**Regulation of the acrAB Transcription**__

__**acrR**__ The acrR gene encodes the AcrR protein, which acts as a repressor of acrAB expression. It is situated at the acr locus of E. coli, but is expressed on a divergant operon, seperate from that expressing acrA and acrB (Ma et al, 1996). The AcrR protein functions as a transcriptional regulator and possesses a helix-turn-helix motif at it's N-terminus.

Surprisingly however, general stress conditions (4% ethanol, 0.5M NaCl, the onset of stationary phase in LB medium) increase the transcription of acrR even more strongly than that of acrAB. This suggests that an additional factor must be responsible for the up-regulation of acrAB in stressful conditions, and that the role of AcrR is to instead function as a secondary modulator to fine tune the level of acrAB transcription and prevent its unwanted over-expression (Ma et al, 1996). Deletion, or inactivation of acrR results in the enhanced expression of acrAb and increases fluoroquinolone resistance in clinical E. coli strains (Jellen-Ritter and Kera, 2001).

Transcription of acrR has also been shown to be autoregulated. That is to say, the AcrR protein has the ability to repress the transcription of the acrR gene, thus repressing its own synthesis (Ma et al, 1996).

__**acrS (envR)**__ The AcrS protein (formerly EnvR) has been shown to repress the expression of the acrAB genes in E. coli (Hirakawa et al, 2008). Indeed, it does so with more efficiency than AcrR. AcrS repression of acrAB is unlikely to be mediated by MarA, SoxS or Rob, since the expression levels of these regulators are not affected by acrS (Hirawaka et al, 2008).

Recent work suggests that acrS may function as a switch for the alternative expression of AcrAB and AcrEF (Hirakawa et al, 2008). AcrEF is a second multi-drug efflux pump, also with a broad substrate range and common physiological roles and features to AcrAB.

__**MarA and SoxS**__ Expression of acrAB can be induced by the global regulators MarA and SoxS. The MarA protein acts as a transcriptional activator in E. coli, and is produced in increasing amounts in response to antibiotic stress (Ma et al, 1996). The soxRS genes form another global regulatory system in E. coli, that leading to the expression of various genes in response to super-oxides. The SoxS protein acts as a transcriptional activator, and it's synthesis is controlled by SoxR.

The fact that acrAB expression can be induced by MarA and SoxS was demonstrated based on the fact that transcription of acrAB is increased in strains of E. coli constitutively expressing MarA and SoxS (Ma et al, 1996).

However, while MarA and SoxS can induce transcription of acrAB, they are not responsible for it's upreguation during general stressconditions. This was demonstrated by the fact that E. coli strains totally lacking MarA and/or SoxS were still able to induce acrAB expression under general stress conditions, implying that another factor must be responsible for this induction.

__**Rob**__ Rob (right oriC binding) is a homologue of MarA and SoxS, and like them it acts as a transcriptional activator. A strain of E. coli that over produces Rob has been shown to have increased levels of acrAB transcription. This suggests that Rob could play a role in regulating acrAB gene expression. __**The Outer Membrane Channel TolC**__

The AcrAB pump exports some beta-lactam antibiotics in the periplasm, out of cells via the outer membrane channel TolC (Nishino et al, 2003).