Regulation of Salmonella enterica Pathogenicity Island 1 by DNA Adenine Methylation

DNA adenine methylase (Dam ) mutants of Salmonella enterica are attenuated in the mouse model and present multiple virulence-related defects. Impaired interaction of Salmonella Dam mutants with the intestinal epithelium has been tentatively correlated with reduced secretion of pathogenicity island 1 (SPI-1) effectors. In this study, we show that S. enterica Dam mutants contain lowered levels of the SPI-1 transcriptional regulators HilA, HilC, HilD, and InvF. Epistasis analysis indicates that Dam-dependent regulation of SPI-1 requires HilD, while HilA, HilC, and InvF are dispensable. A transcriptional hilD::lac fusion is expressed at similar levels in Dam(+) and Dam hosts. However, lower levels of hilD mRNA are found in a Dam background, thus providing unsuspected evidence that Dam methylation might exert post-transcriptional regulation of hilD expression. This hypothesis is supported by the following lines of evidence: (i) lowered levels of hilD mRNA are found in Salmonella Dam mutants when hilD is transcribed from a heterologous promoter; (ii) increased hilD mRNA turnover is observed in Dam mutants; (iii) lack of the Hfq RNA chaperone enhances hilD mRNA instability in Dam mutants; and (iv) lack of the RNA degradosome components polynucleotide phosphorylase and ribonuclease E suppresses hilD mRNA instability in a Dam background. Our report of Dam-dependent control of hilD mRNA stability suggests that DNA adenine methylation plays hitherto unknown roles in post-transcriptional control of gene expression.