We describe the derivation of two strains of Salmonella typhimurium LT2 which are r-m+ for all three of the known chromosomal genes for the restriction and modification of DNA, hsdLT, hsdSA, and hsdSB; the strains were designated LB5000 and LB5010. LB5000 is a smooth derivative sensitive to phage P22; LB5010 is a galE strain sensitive to phage P1.

A recombinant strain, isolated following the transduction of an Escherichia coli recipient carrying the Salmonela typhimurium (SB) specificity genes with DNA from a donor having the Salmonella potsdam (SP) specificity, was shown [Bullas, L. R., Colson, C. & Van Pel, A. (1976)J. Gen. Microbiol. 95, 166-172] 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 around 100 base pairs flanked by two nonhomologous regions each of approximately 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 one nonhomologous region from the SB gene and the other from the SP gene, as predicted from genetic exchange within the conserved sequence. This finding supports the idea that two domains of an hsdS polypeptide, which are different for each specificity, may correlate with two regions of the DNA sequence recognized. It has been shown that the recognition sequences for E. coli K-12 and B each consist of two short oligonucleotide sequences interrupted by a nonspecific sequence. A similar organization is suggested for the SalmoneUa specificity systems, providing the potential for evolutionary diversification of restriction specificities as a result of recombination within the conserved sequence of the hsdS gene.The type I restriction (r) and modification (m) systems characteristic of Escherichia coli K-12 and E. coli B have been studied extensively (for reviews see refs. 1-3). The relevant enzymes EcoK and EcoB interact with a specific DNA sequence and may methylate (modify) this sequence or alternatively cleave (restrict) the DNA at relatively random locations, which may be several thousand base pairs (bp) from the interaction site.The genes encoding the K restriction system are located counterclockwise to serB at 98.5 min on the E. coli K-12 chromosome (4, 5). Complementation tests between mutants defective in the E. coli K-12 and E. coli B restriction systems led to the hypothesis that these systems are encoded by three genes: hsdS (hsd for host specificity determinant), hsdM, and hsdR. The product of hsdS is responsible for recognition of the DNA sequence thus imparting specificity; that of hsdM, together with that of hsdS, is required for modification, while the products of all three genes are essential for restriction (6)(7)(8). Complementation tests that indicated the exchange of subunits between EcoK and EcoB (6, 9) imply that the E. coli K-12 and B restriction systems are related.Restriction and modification systems have been demonstrated in several different Salmonella strains (10, 11), and of these, the genes encoding SB of Salmonella typhimurium (12), like those of E. coli K-12, map in the chromosom...

SUMMARYAs the result of PI-mediated cotransduction with serB from Salmonella potsdam to the Escherichia coli/Salmonella typhimurium hybrid 46 r 7, one recombinant, ~4004, 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 ~4 0 0 4 arose as the result of a recombination event within the hsdS genes of SB and SP is discussed.

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With the use of four different phages, Salmonella strains representing 85 different serotypes were examined to determine their restriction-modification phenotype. They fell into one of three groups on this basis: group 1, those which lacked the common LT system; group 2, those in which only the LT system could be recognized; and group 3. those which possessed the LT system and at least one other system shown with some serotypes to be closely linked to serB. The specificity of the serB-linked restriction-modification system was unique for each serotype, but different strains of the same serotype expressed the same specificity. Two of the systems were shown to behave in genetic crosses as functional alleles of the S. typhimurium SB system. It is possible that these serB-linked restriction-modification systems constitute a large multiallelic series of genes extending throughout the Salmonella genus and Escherichia coli. We suggest that the division of the Salmonella into the three restriction-modification groups may be significant in defining a "biological grouping" of the different serotypes within the genus which may ultimately be useful in describing the Salmonella species. From the genetic relatedness between the genes of some of the Salmonella restriction-modification systems with those of the E. coli systems, we deduce that the restriction endonuclases produced by the Salmonella serB-linked systems are of type 1. Determination of the nucleotide sequences of the recognition sites of the restriction endonucleases of selected Salmonella systems should further our understanding of specificity with these enzymes.