The excision sites

The next step was the precise definition of the sequences involved in the excision process. The basic idea of the deletion model mentioned above was that the instability of the mitochondrial genome of yeast was due to the existence in each genome unit of a number of nucleotide sequences having enough homology to allow illegitimate, unequal recombination to take place. In this respect, clearly the GC clusters were at least as good candidates as the AT spacers. 

Such a simple excision mechanism was not found in petites induced by ethidium bromide, practically the only ones studied in other laboratories. These contained inverted 

It was obvious that what was needed was detailed knowledge, at the nucleotide level, of the sequences involved in the excision of petite genomes. The simplest interpretation of the sequence data was that excision was clue to a crossing-over process, and that the primary event in the spontaneous petite mutation was very similar to the excision of the lambda prophage from the E.coli chromosome, or to (he dissociation of a bacterial transposon from its host plasmid in this case, the GC clusters and sequences in the AT spacers play the same role as the insertion sequences delimiting a bacterial transposon. 

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