LEUCINE-RESPONSIVE REGULATORY PROTEIN AND DEOXYADENOSINE METHYLASE CONTROL THE PHASE VARIATION AND EXPRESSION OF THE SFA AND DAA PILI OPERONS IN ESCHERICHIA-COLI

The Escherichia coli operons daa and sfa encode F1845 and S pill, resp ectively. In this paper we show that the expression of these operons i s under phase variation control at a transcriptional level. The transc ription of both operons is dependent on the global regulator leucine-r esponsive regulatory protein (Lrp) and deoxyadenosine methylase (Dam). Lrp is required for methylation protection of two GATC sites located within conserved DNA sequences in the regulatory regions of these oper ons. These GATC sites are differentially methylated, establishing a me thylation pattern which is characteristic of either the phase ON or ph ase OFF state. We also show that Lrp binds to the daa and sfa regulato ry regions and that this binding is modulated by the methylation of th e GATC sites. These results indicate that the phase variation of the d aa and sfa operons is regulated by a mechanism involving differential binding of Lrp owing to methylation of GATC sites in the regulatory re gion, which is similar to the mechanism that controls phase variation of the pap operon.