AT spacers

It was suggested that the absence of an inverted region for the ET reactions at spacer-covered metal electrodes is due to the availabihty of a continuum of electronic states in metal electrodes below the Fermi level. For the same reason, the inverted region is also not expected to be seen for the homogeneous intermolecular ET reactions because a continuum of electronic states are also available below and above the respective ground states of acceptor and donor ions in solutions involved in homogeneous ET reactions. 

Figure 2,6. A schematic representation of the organization of a mitochondrial genome unit of yeast. Grey stretches represent genes, white stretches AT spacers, black bars GC clusters. (From Bernardi, 1979a).

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. 

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. 

Figure 2.9. Physical and genetical map of the mitochondrial genome unit of wild-type yeast (strain A). Some restriction sites are indicated. Circled numbers indicate the location of ori sequences 1-8 (arrowheads point in the direction cluster C to cluster A sec Fig. 2.8). Black and dotted areas correspond to exons and introns of mitochondrial genes, respectively. Thin radial lines ending in small circles indicate tRNA genes. White areas correspond to long AT spacers embedding short GC clusters. (Modified from de Zamaroezy et al.,...

It should be pointed out that the repeat unit of petite Zl extends 80 bp to the left of cluster A and only 40 bp to the right of sequence r, whereas those of petites 26 and 14 extend 115 bp to the right of sequence r (Fig. 2.18) in all cases, however, these extensions to the left of ori sequences are just made of AT spacer. Effects of flanking regions of ori sequences on the replicative ability of the latter are known (Rayko et al., 1988 see Section 2.5), but they are small compared with the different suppressivities exhibited by petites 14 and 26 relative to petite Zl. There is, therefore, no doubt that these differences are due to the deletions in the ori sequences of petites 14 and 26. 

Intergenic sequences represent 63% of the mitochondrial long (85 kb) genome of S. cerevisiae. They comprise 170-200 AT spacers that correspond to 47% of the genome... 

There are several reports of twins with flexible tail-to-tail linking. In most cases two equal units are linked twins of this kind have been called symmetric [224, 225, 225 a]. Recently a series has been reported in which the lower members are smectic, and only at spacer lengths above six carbon atoms do the compounds appear nematic [225]. In other cases two unequal units are connected, producing a non-symmetric dimer [226-228,228a-b]. There have also been detailed investigations of the flexibility of the linking groups [228, 229], and... 

---