by the telomerase, but exchange has to be considered as partially responsible. These exchanges very likely influence the stability of the terminal regions of the yeast chromosomes (4).
It is clear that the known long repeats of the terminal regions—e.g., Y′, X′, and W′—are an intimate part of the process of terminal region sequence exchange that is probably responsible for the patterns shown in Fig. 3. It seems likely that they originate in this process. That leaves open the question as to whether all of these duplications and multiplications are simply the inevitable result of the process of terminal region sequence exchange. Whether they have useful functions is yet uncertain, though the X2 repeat (Fig. 4) is well conserved and present on every chromosome end. There are no examples of precise and long direct sequence similarity between terminal regions on the opposite ends of chromosomes. This finding suggests that the orientation of these sequences or part of them is important to yeast survival. These sequences point outwards (or inwards depending on point of view) from all yeast chromosomes. They are terminated by the simple sequences generated by telomerase. The telomerase sequences are clearly significant to chromosome stability and replication, but there is good evidence that they carry out other functions. Changing their length affects survival (4).
The central issue is, of course, the evolutionary role and potential function of the reverse complementary relationship of the terminal regions, but little can yet be said. Finally, it seems very unlikely that this pattern of asymmetry is restricted to yeast chromosomes. As the human genome project advances so that sufficient lengths of terminal regions are available it will be interesting to see how well the reverse complementary relationship holds in our own genome. The prediction is that it will be very similar to the yeast situation with allowance for different telomerase synthesized sequences and lengths and distinct sets of repeats in the subterminal regions.
The yeast chromosomal sequences were obtained from the Stanford SGD http://genome-www.stanford.edu/Saccharomyces/. Thanks to Ed Louis for preprints. Johnny Williams prepared useful software in Perl language. This work was supported by National Institutes of Health grants.
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