Type I restriction-modification (R-M) endonucleases are composed of three subunits--HsdR, required for restriction, and HsdM and HsdS which can produce a separate DNA methyltransferase. The HsdS subunit is required for DNA recognition. In this paper we describe the effect of cloned EcoKI and EcoR124I hsd genes on the resulting R-M phenotype. The variability in the expression of the wild type (wt) restriction phenotype after cloning of the wt hsd genes in a multicopy plasmid in Escherichia coli recA+ background suggests that the increased production of the restriction endonuclease from pBR322 is detrimental to the cell and this leads to the deletion of the cloned hsd genes from the hybrid plasmid and/or inactivation of the enzyme. The effect of a mutation in E. coli recA gene on the expression of R-M phenotype is described and discussed in relation to the role of the cell surface and the localization of the restriction endonuclease in the cell.

• MeSH
• Genes, Bacterial MeSH
• Bacterial Proteins genetics   metabolism   MeSH
• DNA Restriction-Modification Enzymes genetics   metabolism   MeSH
• Escherichia coli enzymology   genetics   growth & development   MeSH
• Cloning, Molecular MeSH
• Mutation MeSH
• Plasmids genetics   MeSH
• Escherichia coli Proteins * MeSH
• Rec A Recombinases genetics   MeSH
• Gene Expression Regulation, Bacterial * MeSH
• Deoxyribonucleases, Type I Site-Specific genetics   metabolism   MeSH
• DNA Restriction Enzymes genetics   metabolism   MeSH
• Magnesium Sulfate pharmacology   MeSH
• Temperature MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Bacterial Proteins MeSH
• DNA Restriction-Modification Enzymes MeSH
• endodeoxyribonuclease EcoKI MeSH Browser
• endodeoxyribonuclease EcoR124I MeSH Browser
• HsdR protein, E coli MeSH Browser
• HSDS protein, Bacteria MeSH Browser
• Escherichia coli Proteins * MeSH
• Rec A Recombinases MeSH
• Deoxyribonucleases, Type I Site-Specific MeSH
• DNA Restriction Enzymes MeSH
• Magnesium Sulfate MeSH

Induced mutagenesis was studied in Escherichia coli K12 cells in relation to the level of RecA-protein (P-RecA). In experiments strains AB2497, AB2497(pBR322) and AB2497(pX02) were used. The multicopy plasmid pX02 is a recombinant of pBR322 and recA+ gene of E. coli K12. Cells carrying this plasmid overproduce the P-RecA constitutively. Mutagenesis was induced by the decay of incorporated 6-3H-thymidine. Mutations of the argE3 (ochre) to Arg+ prototrophy were followed. Besides the frequency of mutations, mutagenic specificity was determined. In cells AB2497(pX02) which overproduce the P-RecA the yield of Arg+ revertants was markedly reduced compared with that in strains AB2497 or AB2497(pBR322), whereas the mutagenic specificity was not changed. In all the strains studied the predominant type of mutation produced was the base substitution in the A:T base pair.

• MeSH
• Genes, Bacterial * MeSH
• Escherichia coli genetics   metabolism   MeSH
• Phenotype MeSH
• Mutation * MeSH
• Plasmids * MeSH
• Rec A Recombinases biosynthesis   genetics   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Rec A Recombinases MeSH