Genetic recombination can generate altered restriction specificity.

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Abstract
A recombinant strain, isolated following the transduction of an Escherichia coli recipient carrying the Salmonella typhimurium (SB) specificity genes with DNA from a donor having the S. potsdam (SP) specificity, was shown to have neither SB nor SP specificity but to encode a novel restriction specificity, SQ. The heteroduplex analysis of the hsdS (specificity) genes of the SB and SP restriction and modification systems described here identifies a conserved sequence of .apprx. 100 base pairs flanked by 2 nonhomologous regions each of .apprx. 500 base pairs. This organization parallels that previously deduced from the DNA sequences of the hsdS genes of the related E. coli K-12, B and D restriction systems. The present heteroduplex analyses further show that the hsdS gene conferring the SQ specificity derives 1 nonhomologous region from the SB gene and the other from the SP gene, as predicted from genetic exchange within the conserved sequence. Evidently, 2 domains of an hsdS polypeptide, which are different for each specificity, may correlate with 2 regions of the DNA sequence recognized. It has been shown that the recognition sequences for E. coli K-12 and B each consist of 2 short oligonucleotide sequences interrupted by a nonspecific sequence. A similar organization is suggested for the Salmonella specificity systems, providing the potential for evolutionary diversification of restriction specificities as a result of recombination within the conserved sequence of the hsdS gene.