Sugars that induce the Mlc regulon repress hilD expression by activating hilE 
Sequestration of Mlc by the unphosphorylated form of glucose permease, enzyme IICBGlc, is known to displace Mlc from its DNA-binding sites, thereby allowing transcription of its target genes (25-27).
The Mlc regulon can be induced by glucose and, to a lesser extent, by mannose (39).
The effects of various carbon sources on the expression of hilE and hilD were studied with two Mlc regulon-inducing sugars, glucose and mannose, and two Mlc regulon-non-inducing carbohydrates, arabinose and glycerol.
SL1344 and SR1304 carrying either pMAB69 (hilE-lacZY) or pJB5 (hilD-lacZY) were cultivated in TB that was buffered to pH 7.0 with 0.1M MOPS, to minimize pH effects arising from growth in the presence of the various carbon sources.
In the wild-type strain, glucose and mannose increased hilE expression 2-fold and hilE activation reduced hilD expression (Figure 6).
The expression levels of hilE and hilD remained almost unaffected in the presence of glycerol.
Interestingly, arabinose also activated hilE, albeit to a lesser degree than either glucose or mannose.
Nevertheless, this caused a minute increase in hilD expression.
In the absence of sugars, the mlc mutation increased hilE expression 1.5-fold, with a concomitant reduction in hilD expression.
On the other hand, the expression in SR1304 of both hilE and hilD was increased in the presence of glucose, mannose or arabinose.
These results imply that multiple regulatory pathways are involved in the regulation of hilE and hilD by carbohydrates.
