Defined mutations of acrA or acrB (formerly acrE) genes increased the susceptibility of Escherichia coli to a range of small inhibitor molecules. Deletion of acrAB increased susceptibility to cephalothin and cephaloridine, but the permeability of these beta-lactams across the outer membrane was not increased. This finding is inconsistent with the earlier hypothesis that acrAB mutations increase drug susceptibility by increasing the permeability of the outer membrane, and supports our model that acrAB codes for a multi-drug efflux pump. The natural environment of an enteric bacterium such as E. coli is enriched in bile salts and fatty acids. An acrAB deletion mutant was found to be hypersusceptible to bile salts and to decanoate. In addition, acrAB expression was elevated by growth in 5 mM decanoate. These results suggest that one major physiological function of AcrAB is to protect E. coli against these and other hydrophobic inhibitors. Transcription of acrAB is increased by other stress conditions including 4% ethanol, 0.5 M NaCl, and stationary phase in Luria-Bertani medium. Finally, acrAB expression was shown to be increased in mar (multiple-antibiotic-resistant) mutants.

The DNA fragment containing the acrA locus of the Escherichia coli chromosome has been cloned by using a complementation test (4,24

Genes acrAB encode a multidrug efflux pump in Escherichia coli. We have previously reported that transcription of acrAB is increased under general stress conditions (i.e. 4% ethanol, 0.5 M NaCl, and the stationary phase in Luria-Bertani medium). In this study, lacZ transcriptional fusions and an in vitro gel mobility shift assay have been utilized to study the mechanisms governing the regulation of acrAB. We found that a closely linked gene, acrR, encoded a repressor of acrAB. Nevertheless, the general stress conditions increased transcription of acrAB in the absence of functional AcrR, and such conditions surprisingly increased the transcription of acrR even more strongly than that of acrAB. These results suggest that the general-stress-induced transcription of acrAB is primarily mediated by global regulatory pathway(s), and that one major role of AcrR is to function as a specific secondary modulator to fine tune the level of acrAB transcription and to prevent the unwanted overexpression of acrAB. To our knowledge, this represents a novel mechanism of regulating gene expression in E. coli. Evidence also suggests that the up-regulation of acrAB expression under general stress conditions is not likely to be mediated by the known global regulators, such as MarA or SoxS, although elevated levels of these proteins were shown to increase the transcription of acrAB.

No abstract

Carotenoid pigments are essential for the protection of both photosynthetic and non-photosynthetic tissues from photooxidative damage. Although carotenoid biosynthesis has been studied in many organisms from bacteria to higher plants, little is known about carotenoid biosynthetic enzymes, or the nature and regulation of the genes encoding them. We report here the first DNA sequence of carotenoid genes from any organism. We have determined the complete nucleotide sequence (11,039 bp) of a gene cluster encoding seven of the eight previously known carotenoid genes (crtA, B, C, D, E, F, I) and a new gene, designated crtK, from Rhodobacter capsulatus, a purple non-sulfur photosynthetic bacterium. The 5' flanking regions of crtA, I, D and E contain a highly conserved palindromic sequence homologous to the consensus binding site for a variety of prokaryotic DNA-binding regulatory proteins. This putative regulatory palindrome is also found 5' to the puc operon, encoding the light-harvesting II antenna polypeptides. Escherichia coli-like sigma 70 promoter sequences are located 5' to crtI and crtD, suggesting for the first time that such promoters may exist in purple photosynthetic bacteria. The crt genes form a minimum of four distinct operons, crtA, crtIBK, crtDC and crtEF, based on inversions of transcriptional orientation within the gene cluster. Possible rho-independent transcription terminators are located 3' to crtI, B, K, C and F. The 3' end of crtA may overlap transcription initiation signals for a downstream gene required for bacteriochlorophyll biosynthesis. We have also observed two regions of exceptional amino acid homology between CrtI and CrtD, both of which are dehydrogenases.