Methylation of f1 DNA by a restriction endonuclease from escherichia coli B

The ral gene of phage lambda. III. Interference with E. coli ATP dependent functions

The ral gene of phage lambda has previously been shown to counteract host controlled restriction and to enhance DNA modification in Escherichia coli (Zabeau et al., 1980). The studies presented in this paper demonstrate that although ral plays only a minor role in the lytic development of phage lambda, it counteracts different E. coli functions, which, like the E. coli restriction system, are ATP dependent. First, ral was found to specifically decrease the efficiency of recombination mediated by the RecBC pathway. Secondly, we observed that E. coli strains in which ral is constitutively expressed, exhibit phenotypes analogous to those of strains carrying mutations in the transcription termination factor rho. In addition, in rho deficient strains general recombination and host controlled restriction are both reduced. These findings strongly suggest that ral might be a second anti-termination function, which in contrast to the lambda N gene product directly antagonizes rho.

The ral gene of phage lambda. I. Identification of a non-essential gene that modulates restriction and modification in E. coli

Host controlled restriction in Escherichia coli can be relieved by pre-infecting restricting cells with modified lambda helper phages. This process, in which intact unmodified phage genomes are allowed to escape restriction attack, is mediated by a newly identified lambda function called ral. The ral gene has been located by deletion mapping between cIII and N. Efficient expression of the ral gene requires the product of the regulator gene N. Polyacrylamide gel analysis of the lambda proteins specified by the cIII-N region failed to reveal the product of the ral gene, but demonstrated that protein Ea10 is encoded by a gene located immediately to the left of ral. From these results the map order cIII-Ea10-ral-TL1-N was deduced. Ral specifically alleviates restriction in E. coli K and E. coli B, but does not affect restriction systems EcoRI, EcoRII and EcoP1. In addition, ral enhances the modification activity of the EcoK and EcoB restriction enzymes: we observed that efficient modification of progeny phages obtained by propagating unmodified lambda phages in r-m+ hosts, is dependent upon the presence of ral. We thus conclude that the ral gene product acts by modulating the restriction and modification activities of the type I restriction systems in E. coli, and the possible mechanisms will be discussed.

Structures and mechanisms of DNA restriction and modification enzymes

DNA restriction and modification enzymes are responsible for the hostspecific barriers to interstrain and interspecies transfer of genetic information that have been observed in a variety of bacterial cell types. Although the phenomenon of host specificity was initially observed in the early 1950s (Luria & Human, 1952; Bertani & Weigle, 1953), it was nearly a decade later that Arber and his colleagues accurately predicted the molecular basis of the phenomenon. Their experiments with bacteriophage λ demonstrated that a given host-specificity system imparts a specific modification to the viral DNA, and further, that restriction of DNA lacking the appropriate modification is s consquence of nucleolytic hydrolysis upon entry into the host cell (Arber & Dussoix, 1962; Dussoix & Arber, 1962; Arber, Hattman & Dussoix, 1963).

Isolation and properties of a thermostable restriction endonuclease (ENDO R-Bst1503)

A restriction endonuclease was isolated from Bacillus stearothermophilus1503-4R (Bst1503) and purified to homogeneity. The enzyme required Mg2+ ion as a cofactor. Bst1503 exhibited maximal activity between pH 7.5 and 8.0, between 60 and 65 degrees C, and with about 0.2 mM Mg2+. Bst1503 was not inactivated after exposure at 55 or 65 degrees C for up to 10 h. After 2 h of incubation at 70 degrees C, Bst1503 was inactivated by 65%. Bst1503 was rapidly inactivated at 75 degrees C. A single protein-staining band having a molecular weight of 46,000 was observed when Bst1503 was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme was found to exist in two active forms, the predominating form with an S value of 8.3 (180,000) and the second form with an S value of 5.4 (96,000). No conversion between the 8.3S and 5.4S forms was observed after storage. Bst1503 recognized six sites in TP-1C deoxyribonucleic acid (DNA), one site in pSC101 and simian virus 40 DNAs, and three sites in lambdavir DNA. Bst1503 and BamHI were determined to be isoschizomers. The effect of temperatures on the activity and stability of BamHI was determined.