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 a nd the gene 4 helicase together promote the annealing of homologous re gions of two DNA partners to form a joint molecule and subsequent stra nd transfer, In this reaction T7 gene 2.5 protein is essential for joi nt molecule formation, but is not required for T7 gene 4 protein-media ted strand transfer, T7 gene 4 helicase alone is able to mediate stran d transfer, provided that a joint molecule is available, The present p aper shows that, in addition, strand transfer proceeds at a normal rat e even when both DNA partners contain ultraviolet-induced pyrimidine d imers (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-strande d partner prevents strand transfer, A short insert (37 nt) can be tole rated in either partner, Thus, DNA helicase is able to participate in recombinational DNA repair through its role in strand exchange, provid ing a pathway distinct from nucleotide excision repair.