A chromosome containing HOT1 preferentially receives information during mitotic interchromosomal gene conversion

The recombination-stimulating sequence, HOT1, is derived from yeast ribosomal DNA and corresponds to the sequences required for promotion of transcription by RNA polymerase I. The effect of HOT1 on mitotic interchromosomal gene conversion was examined in diploid strains carrying his4 heteroalleles. When HOT1 is inserted adjacent to both copies of HIS4, the frequency of His+ recombinants is increased approximately 10-fold. When HOT1 is present on only one of the two homologs, recombination is enhanced and the his4 gene on the HOT1-containing chromosome is preferentially converted. In both pairs of his4 heteroalleles examined, HOT1 stimulates conversion of the his4 mutation which is further from the site of HOT1 insertion more than it stimulates conversion of the HOT1-proximal his4 allele. Compared to recombinants isolated from control strains that lack HOT1, HOT1-promoted His+ recombinants are more often homozygous for sequences distal to HIS4. The preferential conversion of sequences on the HOT1-containing chromosome is consistent with the double-strand-gap repair model of recombination and suggests that HOT1-promoted gene conversion initiates with a double-strand break in HOT1-adjacent sequences.