doi:10.1006/scdb.2000.0248, available online at http://www.idealibrary.com on seminars in CELL & DEVELOPMENTAL BIOLOGY, Vol. 12, 2001: pp. 225–237 Transcriptional regulation of multidrug efflux pumps in bacteria Steve Grkovic, Melissa H. Brown and Ronald A. Skurray ∗ As integral membrane proteins demonstrating an extraordi- narily wide substrate range, some degree of regulatory control over the expression of bacterial multidrug-resistance (MDR) transporters is to be expected. Excessive expression could be deleterious, due to direct, physical disruption of membrane integrity, or the unwanted export of essential metabolites, a potential side-effect of their broad substrate specificity. There are limited clues as to the physiological functions of most MDR transporters, but their expression is likely to be up- regulated in response to the presence of natural substrates of these pumps. Thus, it is no surprise that MDR genes are subject to regulation at the local level, consisting of examples of both transcriptional repression and activation by proteins encoded adjacent to that for the transporter. Furthermore, an increasing number of MDR genes have also been found to be controlled by global transcriptional activator proteins. Key words: activators / bacterial multidrug efflux pump / global regulation / multi-ligand binding / repressors c  2001 Academic Press Introduction Initial research on bacterial multidrug-resistance (MDR) transporters predominantly focused on the mechanism by which they export from the cell an impressive array of structurally diverse toxic compounds, energized by the antiport of a proton (H + ) or the hydrolysis of ATP. More recent efforts have also been directed towards understanding the complex regulatory pathways controlling expression From the School of Biological Sciences, Macleay Building A12, University of Sydney, Sydney, New South Wales 2006, Australia. *Corresponding author. E-mail: c 2001 Academic Press 1084–9521 / 01 / 030225+ 13 / $35.00 / 0 of MDR genes, resulting in the elucidation of transcriptional regulation at both the local and global levels. The majority of the regulators identified to date fall into one of several regulatory protein families, despite being from widely different species, a grouping which also shows little correlation with the family of the MDR pump whose expression they control (Table 1). All of the bacterial MDR transporters identified to date whose expression is under the control of a transcriptional regulatory protein are proton-motive force (PMF) dependent exporters and members of either the major facilitator superfamily (MFS) or resistance nodulation division (RND) family (Table 1). However, some members of the above families, and all members of another family that utilizes the PMF, the small multidrug resistance (SMR) family, do not have their synthesis controlled at the transcriptional level. Furthermore, sequencing of entire bacterial genomes has led to the identification of a large number of putative MDR transporter genes and their regulators, the functions of which are yet to be confirmed (see Paulsen et al. 1 for a review of MDR transporters and their families). Studies of several local transcriptional regulators have shown that they act by directly binding to a similar wide range of toxic compounds to that exported by the membrane protein whose expression they control, thereby facilitating the induction of MDR gene expression in response to the presence of diverse toxic substances. However, the large majority of known MDR pump substrates and regulator ligands are synthetic compounds, suggesting further research is required to identify natural substrates and ligands for many of these proteins. To date, it has proven extremely difficult to obtain any structural data for a MDR transporter, or for that matter any integral membrane protein. The multidrug- binding abilities of the soluble cytosolic regulators has allowed this problem to be partly addressed, providing a much more amenable system for study, resulting in the recent publication of detailed X-ray 225