The AT spacers and the var1 genes from the mitochondrial genomes of Saccharomyces cerevisiae and Torulopsis glabrata: evolutionary origin and mechanism of formation.

Intergenic sequences represent 63% of the mitochondrial 'long' (85 kb) genome of Saccharomyces cerevisiae. They comprise 170-200 AT spacers that correspond to 47% of the genome and are separated from each other by GC clusters, ORFs, ori sequences, as well as by protein-coding genes. Intergenic AT spacers have an average size of 190 bp, and a GC level of 5%; they are formed by short (20-30 nt on the average) A/T stretches separated by C/G mono- to trinucleotides. An analysis of the primary structures of all intergenic AT spacers already sequenced (32 kb; 80% of the total) has shown that they are characterized by an extremely high level of short sequence repetitiveness and by a characteristic sequence pattern; the frequencies of A/T isostichs conspicuously deviate from statistical expectations, and exponentially decrease when their (AT + TA)/(AA + TT) ratio, R, decreases. A situation basically identical was found in the AT spacers of the mitochondrial genome (19 kb) of Torulopsis glabrata. The sequence features of the AT spacers indicate that they were built in evolution by an expansion process mainly involving rounds of duplication, inversion and translocation events which affected an initial oligodeoxynucleotide (endowed with a particular R ratio) and the sequences derived from it. In turn, the initial oligodeoxynucleotide appears to have arisen from an ancestral promoter-replicator sequence which was at the origin of the nonanucleotide promoters present in the mitochondrial genomes of several yeasts. Common sequence patterns indicate that the AT spacers so formed gave rise to the var1 gene (by linking and phasing of short ORFs), to the DNA stretches corresponding to the untranslated mRNA sequences and to the central stretches of ori sequences from S. cerevisiae.