Synthesis of FinP RNA by plasmids F and pSLT is regulated by DNA adenine methylation.

DNA adenine methylase mutants of Salmonella typhimurium contain reduced amounts of FinP, an antisense RNA encoded by the virulence plasmid pSLT. Lowered FinP levels are detected in both Dam- FinO+ and Dam- FinO- backgrounds, suggesting that Dam methylation regulates FinP production rather than FinP half-life. Reduced amounts of F-encoded FinP RNA are likewise found in Dam- mutants of Escherichia coli. A consequence of FinP RNA scarcity in the absence of DNA adenine methylation is that Dam- mutants of both S. typhimurium and E. coli show elevated levels of F plasmid transfer. Inhibition of F fertility by the S. typhimurium virulence plasmid is also impaired in a Dam- background.     

Regulation of conjugal transfer by Lrp and Dam methylation in plasmid R100.

Conjugal transfer of the F-like plasmid R100 occurs at higher frequencies in the absence of DNA adenine methylation. Lower levels of R100-encoded FinP RNA were found in a Dam host, suggesting that Dam methylation regulates FinP RNA synthesis. Lack of the leucine-responsive regulatory protein (Lrp) causes a decrease in R100 plasmid transfer, indicating that Lrp is an activator of R100-mediated conjugation. Hence, host-encoded regulators previously described for the Salmonella virulence plasmid (pSLT) seem to play analogous roles in R100. Repression of conjugal transfer in rich medium is an additional trait shared by R100 and pSLT. DNA sequence comparisons in regulatory loci support the view that R100 and pSLT are closely related.     

H-NS Can Facilitate Specific DNA-binding by RNA Polymerase in AT-rich Gene Regulatory Regions

Extremely AT-rich DNA sequences present a challenging template for specific recognition by RNA polymerase. In bacteria, this is because the promoter −10 hexamer, the major DNA element recognised by RNA polymerase, is itself AT-rich. We show that Histone-like Nucleoid Structuring (H-NS) protein can facilitate correct recognition of a promoter by RNA polymerase in AT-rich gene regulatory regions. Thus, at the Escherichia coli ehxCABD operon, RNA polymerase is unable to distinguish between the promoter −10 element and similar overlapping sequences. This problem is resolved in native nucleoprotein because the overlapping sequences are masked by H-NS. Our work provides mechanistic insight into nucleoprotein structure and its effect on protein-DNA interactions in prokaryotic cells.