FINE-RESOLUTION ANALYSIS OF PRODUCTS OF INTRACHROMOSOMAL HOMEOLOGOUS RECOMBINATION IN MAMMALIAN-CELLS

Mouse Ltk(-) cell lines that contained a herpes simplex virus type 1 ( HSV-1) thymidine kinase (tk) gene with a 16-bp insertion mutation link ed to either a defective HSV-2 tk gene or a hybrid tk sequence compris ed of HSV-1 and HSV-2 tk sequences were constructed. HSV-1 and HSV-2 t k genes have 81% nucleotide identity and hence are homeologous, Correc tion of the insertion mutant HSV-1 tk gene via recombination with the hybrid tk sequence required an exchange between homeologous tic sequen ces, although recombination could initiate within a region of signific ant sequence identity. Seven cell lines containing linked HSV-1 and HS V-1-HSV-2 hybrid tk sequences gave rise to tk(+) segregants at an aver age rate of 10(-8) events per cell division, DNA sequencing revealed t hat each recombinant from these lines displayed an apparent gene conve rsion which involved an accurate transfer of an uninterrupted block of information between homeologous tk sequences, Conversion tract length s ranged from 35 to > 330 bp. In contrast, cell lines containing linke d HSV-1 and HSV-2 rk sequences with no significant stretches of sequen ce identity had an overall rate of homeologous recombination of < 10(- 9). One such cell line produced homeologous recombinants at a rate of 10(-8). Strikingly, all homeologous recombinants from this latter cell line were due to crossovers between the HSV-1 and HSV-2 tk genes, Our results, which provide the first detailed analysis of homeologous rec ombination within a mammalian genome, suggest that rearrangements in m ammalian genomes are regulated by the degree of sequence divergence lo cated at the site of recombination initiation.