Gene 4 helicase of bacteriophage T7 mediates strand transfer through pyrimidine dimers, mismatches, and nonhomologous regions.
In bacteriophage T7 the gene 2.5 single-stranded DNA-binding protein and the gene 4 helicase together promote the annealing of homologous regions of two DNA partners to form a joint molecule and subsequent strand transfer. In this reaction T7 gene 2.5 protein is essential for joint molecule formation, but is not required for T7 gene 4 protein-mediated strand transfer. T7 gene 4 helicase alone is able to mediate strand transfer, provided that a joint molecule is available. The present paper shows that, in addition, strand transfer proceeds at a normal rate even when both DNA partners contain ultraviolet-induced pyrimidine dimers (0.6 dimer per 100 nt). An insert of a relatively long (842-nt) segment of nonhomologous DNA in the single-stranded DNA partner has no effect on strand transfer, whereas its presence in the double-stranded partner prevents strand transfer. A short insert (37 nt) can be tolerated in either partner. Thus, DNA helicase is able to participate in recombinational DNA repair through its role in strand exchange, providing a pathway distinct from nucleotide excision repair.
Kong
D
D
Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115, USA.
Griffith
J D
JD
Richardson
C C
CC
D017123:Bacteriophage T7 / Q000201:enzymology*; D019915:DNA Primase; D008854:Microscopy, Electron; D009692:Nucleic Acid Heteroduplexes; D011740:Pyrimidine Dimers / Q000378:metabolism*; D012316:RNA Nucleotidyltransferases / Q000378:metabolism*
D009692:Nucleic Acid Heteroduplexes; D011740:Pyrimidine Dimers; D019915:DNA Primase; D012316:RNA Nucleotidyltransferases