Complex transcriptional organization regulates an Escherichia coli locus implicated in lipopolysaccharide biogenesis

The Escherichia coli yrbG-lptB locus (yrbG kdsD kdsC lptC lptA lptB) encodes genes for outer membrane biogenesis, namely, kdsC and kdsD for biosynthesis of the lipopolysaccharide inner core sugar Kdo, and lptA, lptB, and lptC for lipopolysaccharide transport to the outer membrane. Three promoters (yrbGp, kdsCp and the σE-dependent lptAp) have been previously identified by genetic analysis. In this work, we show that transcription of this locus generates an array of overlapping mRNAs and we characterize the two intralocus promoter regions. In the kdsCp region, we identified three promoters (kdsCp1, kdsCp2, and kdsCp3) scattered within about 600nt in the 3′-coding region of kdsD. The lptAp region is composed of two closely spaced promoters, lptAp1 and lptAp2. The former had been previously identified as a σE-dependent promoter. Interestingly, lptAp1 is not activated by several stressful conditions that normally induce the σE-dependent envelope stress response, whereas it seems to respond to conditions affecting lipopolysaccharide biogenesis, thus implying a specialized σE-dependent LPS stress signaling pathway.