Excision and recombination

These results provided the first evidenee for physical recombination of mitochondrial DNA in crosses of wild-type yeast eells, indieated that recombination was very frequent in crosses and shed some light on mitochondrial segregation. They also provided examples of reeombination phenomena in an interspersed system of unique and repetitive nucleotide sequences. 

The canonical and the. surrogare origins of replication of petite genomes 

It should be noted (i) that some ori sequences display one orientation and some the opposite one on the wild-type genome (ii) that ori 2 and 7 as well as ori 3 and 4 are close to each other and tandemly oriented (iii) that ori 4 is absent in a wild-type strain (iv) that ori sequences containing the 7 cluster were found only once (ori 4), or not at all (ori 6, ori 7) in extensive screenings of spontaneous petite genomes (see Table 2.1). 

Ori° petites, lacking a canonical ori sequence, were also found, although very rarely. An investigation of the mitochondrial genomes of eight such ori° petites (Goursot et al., 1982) has revealed that their repeat units contain, instead of canonical ori sequences, one or more orf or ori surrogate sequences. These orP sequences are a subset of GC clusters char- 

C°CCCACCCcffcCCi cT/ hrAirmATTp ATTACTIArATATTl[AlrAAATATAAATlcTMCrTAArTAATAArT iitinAATATAl TTAATATTrAA 

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Cole, R.S. (1973) Repair of DNA containing interstrand crosslinks in Escherichia coli sequential excision and recombination. Proc. Natl. Acad. Sci. USA, 70, 1064-1068. 

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