Initiation of recA+-dependent recombination in Escherichia coli (lambda) . II. Specificity in the induction of recombination and strand cutting in undamaged covalent circular bacteriophage 186 and lambda DNA molecules in phage-infected cells

Ross, P.; Howard-Flanders, P.

Journal of Molecular Biology 117(1): 159-174

1977


ISSN/ISBN: 0022-2836
PMID: 340700
DOI: 10.1016/0022-2836(77)90029-8
Accession: 068526615

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Abstract
Covalent circular .lambda. DNA molecules produced in E. coli (.lambda.) host cells by infection with labeled .lambda. baceriophages are cut following superinfection with .lambda. phages damaged by exposure to psoralen and 360 nm light. This cutting of undamaged covalent circular molecules is referred to as cutting in trans, and could be a step in damage-induced recombination. Similar experiments performed with the temperate phage 186, which is not homologous with phage .lambda., showed cutting in trans and damage-induced recombination to occur in homoimmune crosses with phage 186 also. Double lysogens carrying both .lambda. and 186 prophages were used in a test for specificity in cutting in trans and in damage-induced recombination. The double lysogens were infected with 3H-labeled 186 and 32P-labeled .lambda. phages. When these doubly infected lysogens containing covalent circular phage DNA molecules of both types were superinfected with psoralen-damaged 186 phages and incubated, the covalent circular 186 DNA was cut, while .lambda. DNA remained intact. Similarly, superinfection with damaged .lambda. phages caused .lambda., but not 186, DNA to be cut. Evidently, cutting in trans was specific to the covalent circular DNA homologous to the DNA of the damaged phages. Homoimmune phage-prophage genetic crosses were performed in the double lysogenic host infected with genetically marked .lambda. and 186 phages. Damaged-induced recombination was observed in this sytem only between the damaged phage DNA and the homologous prophage, none being detected between other homolog pairs present in the same cell. The damaged phage DNA probably does not induce a general state of enhanced strand cutting and genetic recombination affecting all homolog pairs present in the host cell. The simplest interpretation of the specificity in cutting and in recombination is as follows. When they were incised, the damaged phage DNA molecules are able to pair directly with their undamaged covalent circular homologs. The latter molecules are cut in a recA+-dependent reaction by a recombination endonuclease that cuts the intact member of the paired homologs.