Abstract
The yeast mitochondrial genome contains three reading frames, RF1, RF2, and RF3, which are related to the group I maturases, though they are not intronic sequences. In the Saccharomyces cerevisiae strain D273-10B/A, these reading frames are interrupted by G + C-rich sequences (GC clusters) which break the frames. In the present work we described a Saccharomyces uvarum strain which possesses a RF3 continuous sequence devoid of GC clusters. Moreover, our results strongly suggest that in the same strain RF2 and RF1 are also continuous sequences. As all three RFs belong to transcription units which are highly expressed, it is most reasonable to assume that RF1, RF2, and RF3 are functional genes. Furthermore, we have discovered a rule which seems to explain the transposition of GC clusters, considered as mobile elements, in the mitochondrial genome.
Highlights
The yeast mitochondrialgenomecontains three reading frames, RF1, RF2, and RF3, which are related to the groupI maturases, though they are not intronic sequences
In the present work we described a Saccharomyces uvarum strain which possesses a RF3 continuous sequence devoid of GC clusters
Our results strongly suggest that in the same strain RF2 and RF1 are continuous sequences devoid of GC clusters.As all three RFs belong to transcription unitswhich are highly expressed (Simon and Faye, 198413; Thalenfeld et al, 1983; Coruzzi et al, 1981) it seems most reasonable to assume that RF1, RF2, and RF3 are functionalgenes
Summary
Southern Blots-The mitochondrial DNA of 20 laboratory yeast strains from different sources (Table I) was analyzed in order to know whether RF1, RF2, or RF3 sequences devoid of GC clusterscouldbefound.Thetotal. The probe used, to reveal the Southern blots, was the mitochondrial DNA from the rho- strain DS302 (Coruzzi et al, 1981).The autoradiogram is shown in F2ig. Saccharomyces uuarum (lane 5 ) is the most interesting; there is no HpaII site in its RF1 open frame. This means that it is not interrupted by a GC cluster since three HpaII sites are found in the corresponding G+C-rich sequenocfeD273-10Bl. The RF2 region was analyzed by digesting the DNA preparations with EcoRI, PuuIaIn, d MboI (Fig. 3). S. uuarum (lane 5 , pattern C) is the sole strain in which bands corresponding to the 698a-nd 468-bplong fragments showa small reduction in size suggesting the absence of GC clusters in its RF2 open reading frame.
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