Salmonella typhimurium SA host specificity system is based on deoxyribonucleic acid-adenine methylation

Abstract

We have determined the nature of the deoxyribonucleic acid (DNA) modification governed by the SA host specificity system of Salmonella typhimurium. Two lines of evidence indicate that SA modification is based on methylation of DNA-adenine residues. (i) The SA+ locus of Salmonella was transferred into Escherichia coli B, a strain that does not contain 5-methylcytosine in its DNA; although the hybrid strain was able to confer SA modification, its DNA still did not contain 5-methylcytosine. (ii) the N6-methyladenine content of phage L DNA was measured after growth in various host strains; phage lacking SA modification contained fewer Nf6-methyladenine residues per DNA. We also investigated the possibility, suggested by others (32), that SA modification protects phage DNA against restriction by the RII host specificity system. Phages X, P3, and L were grown in various SA+ and SA-hosts and tested for their relative plating ability on strains containing or lacking RII restriction; the presence or absence of SA modification had no effect on RII restriction. In vitro studies revealed, however, that Salmonella DNA is protected against cleavage by purified RII restriction endonuclease (R EcoRII). This protection is not dependent on SA modification; rather, it appears to be due to methylation by a DNA-cytosine methylase which has overlapping specificity with the RH modification enzyme, but which is not involved in any other known host specificity system.Deoxyribonucleic acid (DNA) restriction and modification (host specificity) is widespread in bacteria (1,2,4,28). This phenomenon has been shown to be due to the action of two enzyme activities, viz., a modification methylase, which adds methyl groups to adenine or cytosine residues contained in a specific DNA sequence(s), and a restriction endonuclease, which makes double-stranded cleavages in any duplex DNA lacking the appropriate methylation. The methylated bases produced by modification are N6-methyladenine (MeAde) or 5methylcytosine (MeC). The restriction and modification activities may be associated together in a single large multifunctional complex (type I) or as separate, independent molecular species (type II) (6).In Salmonella typhimurium, there are three known distinct restriction-modification systems: (i) LT, which maps near the proC locus (11); (ii) SA (previously designated S [9, 10]), which maps near the serB locus, and (iii) SB, which maps between SA and serB (10). The LT modification appears to be due to adeninespecific methylation (18). Since the SB modification system is allelic with the Escherichia coli B and K systems (34), it is likely that SB modification is based on adenine methylation. Slocum and Boyer (32) suggested that S-(or SA)-specific modification might be mediated by cytosine-specific methylation. This was based on their observation that after growth in an E. coli hybrid inheriting the S. typhimurium SA locus, phage X was completely resistant to RII restriction. Since the RII modification enzyme is a DNA-cytosine methylase (5, 30), it was prop...

Deoxyribonucleic Acid Adenine and Cytosine Methylation in Salmonella typhimurium and Salmonella typhi

“…From these results it is proposed that S. typhimurium and S. typhi have an enzyme which methylates the second cytosine at the sequence -CCQA)GGand which is similar to the methylase of E. coli K-12 coded for by the gene dcm'. The presence of a dcm-like methylase in S. typhimurium has also been suggested by Hattman et al (17).…”

“…the modification enzyme of the K host specificity system accounts for only 2 to 3% of the total 6-MeA present in the E. coli chromosome (23). In S. typhimurium LT2, a higher percentage of 6-MeA might be involved in the L, SA, and SB host specificity systems (14,17), whereas in S. typhi these systems have not been studied in detail.…”

Linkage map of Salmonella typhimurium, edition V.

“…F-prime factors carrying S. typhimurium genes have not yet been isolated for all regions of the map of S. typhimuriun, so S. typhinuriun strains carrying F-prime factors containing E. coli genes (Table 3) have been used in a variety of studies, including: (i) complementation tests for allelism and, consequently, hints concerning linkage map position; (ii) dominance studies (190,497,657); (iii) diagnosis of enzyme quaternary structure (374); (iv) gene dosage studies (142, 657); (v) introduction of suppressors with known modes of action (406,430); (vi) description of Salmonella mutation type or of suppressor gene type (70, 188, 512; F. Winston and D. Botstein, personal communication); (vii) construction of P22 specialized transducing phages (257, 319); and (viii) detection of map locations of E. coli genes via gene dosage (422a) or electrophoretic mobility (234a) of the gene product in Salmonella. Low-efficiency transfer of some E. coli F-prime factors into S. typhimurium is due mainly to several restriction (hsd) barriers (26,107,108,137,237,455,569,632). The restriction can be lessened through admixture of a third, intermtediate, S. typhimurium host defective in restriction (e.g., strain SB5404 hisI4O4 hsd proC51 rpsL667 plated on media lacking histidine and proline [B.…”

Absence in Bacillus subtilis and Staphylococcus aureus of the sequence-specific deoxyribonucleic acid methylation that is conferred in Escherichia coli K-12 by the dam and dcm enzymes

“…cytosine (24). The existence of two methylases of similar specificity has been demonstrated in two bacterial strains closely related to E. coli, Salmonella typhi and Salmonella typhimurium (10,15).…”