Negative control of fae (K88) expression by the 'global' regulator Lrp is modulated by the 'local' regulator FaeA and affected by DNA methylation.

Expression of the K88 (fae) operon is negatively controlled by the co-operative binding of Lrp and FaeA to the fae regulatory region and is dependent on the methylation status of three GATC sites present in this region. In this paper, we describe the binding of Lrp to a T-rich DNA helix between GATC site I and site II. FaeA stabilized and modified the Lrp binding, thereby extending the Lrp footprint over GATC site I and site III. Methylation of GATC site I prevented the binding of Lrp/FaeA at this site and appeared to be essential for the cells, since mutation of this site into GTTC resulted in a lethal overproduction of K88 fimbriae. Methylation of GATC site II and site III reduced the stability of Lrp/FaeA binding. Moreover, methylation of GATC site III stimulated faeB promoter activity. The plasmid population in cells harbouring multiple copies of a K88 plasmid consisted of two differentially methylated forms. Form A plasmids with a methylated GATC site I and site III and a nonmethylated site II (+,-,+) represented 20% of the population and were responsible for high-level expression. Form B plasmids with a methylated GATC site I and a non-methylated site II and site III (+,-,-) represented 80% of the population and were responsible for low-level expression. Apparently, K88 fimbriae expression in vivo is balanced at its maximal possible level by modulation of the methylation status of GATC site III. The ratio (1:4) between these populations is stabilized by a constitutive synthesis of FaeA resulting from the presence of an IS1 insertion upstream of faeA. This IS1 insertion separates the faeA promoter from the FaeB-binding sites, thereby neutralizing the control by FaeB activity on expression of FaeA. Instead, faeA transcription is stimulated by binding of FaeA to the faeA promoter region.