Differential regulation by magnesium of the two MsbB paralogs of Shigella flexneri

Shigella flexneri, a gram-negative enteric pathogen, is unusual in that it contains two nonredundant paralogous genes that encode the myristoyl transferase MsbB (LpxM) that catalyzes the final step in the synthesis of the lipid A moiety of lipopolysaccharide. MsbB1 is encoded on the chromosome, and MsbB2 is encoded on the large virulence plasmid present in all pathogenic shigellae. We demonstrate that myristoyl transferase activity due to MsbB2 is detected in limited magnesium medium, but not in replete magnesium medium, whereas that due to MsbB1 is detected under both conditions. MsbB2 increases overall hexa-acylation of lipid A under limited magnesium conditions. Regulation of MsbB2 by magnesium occurs at the level of transcription and is dependent on the conserved magnesium-inducible PhoPQ two-component regulatory pathway. Direct hexanucleotide repeats within the promoter upstream of msbB2 were identified as a putative PhoP binding site, and mutations within the repeats led to diminished PhoP-dependent expression of a transcriptional fusion of lacZ to this promoter. Thus, the virulence plasmid-encoded paralog of msbB is induced under limited magnesium in a PhoPQ-dependent manner. PhoPQ regulates the response of many Enterobacteriaceae to environmental signals, which include modifications of lipid A that confer increased resistance of the organism to stressful environments and antimicrobial peptides. The findings reported here are the first example of gene duplication in which one paralog has selectively acquired the mechanism for differential regulation by PhoPQ. Our findings provide molecular insight into the mechanisms by which each of the two MsbB proteins of S. flexneri likely contributes to pathogenesis.