The divergent malX and malI promoters share a common DNA site for CRP. As for other divergent bacterial promoters that share an activator-binding site, activation in one direction is largely independent of activation in the opposite direction and this is likely to be due to the low frequency of initiation at most promoters (El-Robh & Busby, 2002). Although the malX and malI promoters share a DNA site for CRP,

each has a separate and independent DNA site for MalI. The malX promoter MalI operator is located upstream of the transcript start and overlaps the upstream end of the −10 hexamer, while the check details malI promoter MalI operator is located downstream of the transcript start. This organization is well conserved in the genomes of different strains of E. coli and related Shigella. Figure 3 shows a comparison of the base sequences upstream of the malX and malI translation start sites in these genomes, and the comparison emphasizes how the precise locations of −10 elements and MalI operator sequences have been maintained. This provides yet another example of how efficient repression can result from a repressor interacting

at different locations at a bacterial promoter (Rojo, 2001; Barnard et al., 2004). Interestingly, repression is marginally greater at the malX promoter than at the malI promoter, and this is consistent with MalI action at the malI promoter being autoregulatory. The E. coli K-12 malX-malI Tyrosine-protein kinase BLK intergenic regulatory region provides a simple example of ‘evolution and tinkering’ (Jacob, 1977). The malX promoter is an unremarkable Sorafenib molecular weight CRP-dependent

promoter that resembles scores of Class II promoters (Busby & Ebright, 1999) and it can be shut off by MalI. In contrast, although the divergent malI promoter resembles a Class II CRP-dependent promoter, it has adapted to ensure that the MalI repressor is always made. Thus, MalI-dependent repression is marginally less efficient compared with the malX promoter, the dependence on CRP is relaxed by the DNA site for CRP being located at position −43.5, and the promoter carries seven repeats of a 5′-TAN8-3′ motif, to facilitate RNA polymerase recruitment (Lloyd et al., 2008). This work was funded by a Wellcome Trust program grant. We thank undergraduate project students, Clare Mensley, James Fuller, and Maria Jesus Pina, for some of the constructions. “
“Molecular ecology methods are now well established for the culture-independent characterization of complex bacterial communities associated with various environmental and animal habitats and are revealing the extent of their diversity. By comparison, it has become clear that only a small minority of microorganisms are readily cultivated in vitro, with the majority of all bacteria remaining ‘unculturable’ using standard methods.