DNA restriction and modification systems in Salmonella. SQ, a new system derived by recombination between the SB system of Salmonella typhimurium and the SP system of Salmonella potsdam.

As the result of P1-mediated cotransduction with serB from Salmonella potsdam to the Escherichia coli/Salmonella typhimurium hybird 4617, one recombinant, L4004, was isolated which had a restriction-modification (R--M) system different from the SB and SP systems of its parents, and was designated SQ. The genes of SQ were allelic to those of the SB system of S. typhimurium and were shown by complementation experiments to be functionally related to those of the K system of E. coli. Evidence that the SQ system in L4004 arose as the result of a recombination event within the hsdS genes of SB and SP is discussed.