FIG. 3.

(A) Southern blot analysis of DNA extracted from strain MK181 at various times after transfer to galactose-containing medium. The DNA was digested with ClaI and probed with a fragment of chromosome II carrying the LYS2 gene. The reference band represents the LYS2 copy on chromosome XV, which is used to quantitate the amount of DNA in each lane. The band designated parental + ura3 GC represents the parental ura3 allele on chromosome II and the product of gene conversion with its allele on chromosome V. The parental band can be separated from the gene conversion product by digesting the DNA with EcoRI or BamHI, since these sites are transferred to the broken chromosome during the ura3 gene conversion event (data not shown). The band designated LYS2 GC represents the product of gene conversion events with LYS2, whereas the bands designated LYS2 GC + CO represent events in which the gene conversion with LYS2 was accompanied by a crossing over. (B) Kinetics of DSB repair. The Southern blot shown in panel A was scanned; the relative intensity of each band after normalization with the reference band is presented. The apparent delay in the appearance of ura3 recombinants compared to LYS2 recombinants is probably due to a technical reason: for the sample taken 4 h after transfer to galactose, the band representing gene conversion with ura3 cannot be distinguished from the background.