The plasmid-stabilizing ytl2 protein coats DNA in a sequence-independent manner

Plasmids carrying the ytl2 gene from the large resident plasmid pSLT of Salmonella typhimurium were stabilized >10(5)-fold (compared to control ytl2-free plasmids) in S. typhimurium cells. Purified Ytl2 protein was localized in the cell cytosol and bound to DNA in a sequence-independent manner to form a high-molecular-weight complex, suggesting cooperative binding to the DNA. A mutant ytl2 gene, with a modified C-terminus, did not mediate plasmid stabilization and the mutant Ytl2 protein did not bind cooperatively to DNA. In vivo, while a plasmid carrying the ytl2 gene was stabilized, another plasmid (lacking ytl2) coexisting in the same cell was not. This result suggests that the Ytl2 protein, newly synthesized in a transcription-translation complex, binds preferentially to DNA of the replicon which encodes it and that this binding initiates subsequent cooperative DNA coating by more Ytl2 molecules. Plasmid-encoded newly synthesized Ytl2 protein is thus unavailable to stabilize coresident plasmid DNA, which does not contain the ytl2 gene.