H. A. Rosenthal

Active protection by bacteriophages T3 and T7 againstE. coli B-and K-specific restriction of their DNA

SummaryThe bacteriophages T3 and T7 are not modified and restricted byE. coli strains with different host specificity (E. coli B, K, O) in vivo. The phages code for a gene product with the ability toovercomeclassicalrestriction (ocr):ocr− mutants are subject to modification and restriction via DNA methylation vs cleavage. The T3 genome possesses recognition sites for the restriction endonuclease R.EcoB which, unless the DNA is B-specifically modified, trigger 5–7 DNA cleavages. Theocr gene function of T3 and T7 is located within the gene 0.3 region of these phages and is not identical with thesam (SAMase) function of T3. The mechanisms ofocr protection remains unclear, while it is certain that this protection by the gene 0.3 protein is exerted in the infected cell and not through “over-all” modification in the preceding growth cycle of the phage.

Protection of Foreign DNA Against Host-Controlled Restriction in Bacterial Cells

SummaryForeign F′lac plasmid DNA which is introduced into potentially restricting E. coli recipient cells can be protected from restriction by preinfecting the recipient cells with UV-inactivated T3 or T7 bacteriophages which express the ocr gene function. The recipient cells survive and are able to replicate themselves as well as the newly acquired plasmid.

Untersuchungen über Phagenlysozym

SummaryThe structural basis of the cold-sensitive phenotype of the mutant cseBU56 is the amino acid substitution tyrosine 88 → histidine, resulting from a point mutation in the lysozyme gene of phage T4D. This was demonstrated by peptide mapping, taking into account the mechanism of bromouracil mutagenesis.The effect of the amino acid substitution is discussed: A direct relationship exists between structural alteration, increased energy of activation of lysozyme reaction and elevated minimal temperature of growth of the cs-mutant compared to that of the wild type.