Host specificity and fertility in Salmonella typhimurium LT7

ATP-dependent restriction enzymes

The phenomenon of restriction and modification (R-M) was first observed in the course of studies on bacteriophages in the early 1950s. It was only in the 1960s that work of Arber and colleagues provided a molecular explanation for the host specificity. DNA restriction and modification enzymes are responsible for the host-specific barriers to interstrain and interspecies transfer of genetic information that have been observed in a variety of bacterial cell types. R-M systems comprise an endonuclease and a methyltransferase activity. They serve to protect bacterial cells against bacteriophage infection, because incoming foreign DNA is specifically cleaved by the restriction enzyme if it contains the recognition sequence of the endonuclease. The DNA is protected from cleavage by a specific methylation within the recognition sequence, which is introduced by the methyltransferase. Classic R-M systems are now divided into three types on the basis of enzyme complexity, cofactor requirements, and position of DNA cleavage, although new systems are being discovered that do not fit readily into this classification. This review concentrates on multisubunit, multifunctional ATP-dependent restriction enzymes. A growing number of these enzymes are being subjected to biochemical and genetic studies that, when combined with ongoing structural analyses, promise to provide detailed models for mechanisms of DNA recognition and catalysis. It is now clear that DNA cleavage by these enzymes involves highly unusual modes of interaction between the enzymes and their substrates. These unique features of mechanism pose exciting questions and in addition have led to the suggestion that these enzymes may have biological functions beyond that of restriction and modification. The purpose of this review is to describe the exciting developments in our understanding of how the ATP-dependent restriction enzymes recognize specific DNA sequences and cleave or modify DNA.

Transfer of the genes for the StyLTI restriction-modification system of Salmonella typhimurium to strains lacking modification ability results in death of the recipient cells and degradation of their DNA

The genes encoding the restriction-modification system StyLTI of Salmonella typhimurium were inserted in vivo into the conjugative plasmid pULB21. This allowed us to transfer the StyLTI genes at a very high frequency and to monitor the fate of recipient cells after mating. Transfer of the StyLTI restriction and modification genes into a modificationless recipient was lethal and resulted in degradation of the cell's DNA. This indicates that, in contrast to any other known restriction-modification systems, StyLTI cannot be established after horizontal transfer into a naive host.

Two-step cloning and expression in Escherichia coli of the DNA restriction-modification system StyLTI of Salmonella typhimurium

The StyLTI restriction-modification system is common to most strains of the genus Salmonella, including Salmonella typhimurium. We report here the two-step cloning of the genes controlling the StyLTI system. The StyLTI methylase gene (mod) was cloned first. Then, the companion endonuclease gene (res) was introduced on a compatible vector. A strain of S. typhimurium sensitive to the coliphage lambda was constructed and used to select self-modifying recombinant phages from a Res- Mod+ S. typhimurium genomic library in the lambda EMBL4 cloning vector. The methylase gene of one of these phages was then subcloned in pBR328 and transferred into Escherichia coli. In the second step, the closely linked endonuclease and methylase genes were cloned together on a single DNA fragment inserted in pACYC184 and introduced into the Mod+ E. coli strain obtained in the first step. Attempts to transform Mod- E. coli or S. typhimurium strains with this Res+ Mod+ plasmid were unsuccessful, whereas transformation of Mod+ strains occurred at a normal frequency. This can be understood if the introduction of the StyLTI genes into naive hosts is lethal because of degradation of host DNA by restriction activity; in contrast to most restriction-modification systems, StyLTI could not be transferred into naive hosts without killing them. In addition, it was found that strains containing only the res gene are viable and lack restriction activity in the absence of the companion mod gene. This suggests that expression of the StyLTI endonuclease activity requires at least one polypeptide involved in the methylation activity, as is the case for types I and III restriction-modification systems but not for type II systems.

Identification of the recognition sequence for the M.StyLTI methyltransferase of Salmonella typhimurium LT7: an asymmetric site typical of type-III enzymes

The StyLTI restriction-modification (R-M) system is encoded by chromosomal genes of Salmonella typhimurium LT7. We report here the identification of the nucleotide (nt) sequence methylated by the StyLTI modification methyltransferase (M.StyLTI). This enzyme was partially purified from an Escherichia coli strain expressing the cloned M.StyLTI-encoding gene, but lacking StyLTI restriction activity, and used to methylate DNAs of known sequence, using S-adenosyl-[methyl-3H]-methionine as the methyl donor. The [3H]methylated DNA was then digested with various endonucleases. Examination of labelled and unlabelled restriction fragments allowed us to map the M.StyLTI sites in perfectly defined regions of the DNA. Comparison of the nt sequences of DNA segments with or without M.StyLTI sites permitted us to identify the asymmetric and nondegenerate pentanucleotide, 5'-CAGAG-3', 3'-GTCTC-5' as the StyLTI sequence. M.StyLTI was found to methylate only the 3' A (see asterisk) in the upper strand of this sequence. Thus, M.StyLTI recognises and methylates the DNA in a manner very similar to that of the three known type-III MTases, M.EcoPI, M.EcoP15, and M.HinfIII. This strongly suggests that StyLTI constitutes a fourth type-III R-M system.

Nucleoside diphosphate sugar hydrolase gene of Salmonella typhimurium: chromosomal location determined by intergeneric crosses

The gene specifying a membrane-bound nucleoside diphosphate sugar hydrolase of Salmonella typhimurium was mapped near the metA locus by using intergeneric crosses between this bacterium and Escherichia coli.

Methylation of cytosine residues in DNA controlled by a drug resistance factor (host-induced modification-R factors-N 6 -methyladenine-5-methylcytosine)

The proportion of 5-methylcytosine (5MeCyt) and 6-methylaminopurine (N(6)-methyladenine, 6MeAde) in bacteriophage P22 DNA was analyzed as a function of the host-specificity the phage carried. In the DNA of P22 grown in strains harboring the modifying drug-resistance-transfer-factor N-3, the 5MeCyt content was at least twice that after growth in strains lacking the factor. In contrast, the 6MeAde level of P22 DNA was unaffected by the presence or absence of the factor. The 6MeAde and 5MeCyt levels were unaffected by factors 222 and N-1, which do not modify phage DNA. The 5MeCyt/6MeAde ratio was only slightly higher in the DNA of Salmonella strains that had received the N-3 factor. After transfer of the N-3 factor to Escherichia coli strain B, which normally lacks 5MeCyt, a high content of 5MeCyt is observed. We conclude that the N-3 factor controls a DNA methylase specific for cytosine residues. If the N-3 host specificity is imparted by cytosine methylation, this would be the first instance where a biological role for 5MeCyt has been elucidated.

Variation of 6-methylaminopurine content in bacteriophage P22 deoxyribonucleic acid as a function of host specificity

The 6-methylaminopurine (MAP) content of P22 deoxyribonucleic acid has been analyzed as a function of the host specificity it carries. A 40 to 50% reduction in MAP level occurs as a result of growth in host cells defective in the ability to confer LT specificity.