Gene density near the ends of Saccharomyces cerevisiae chromosomes is much lower than on the rest of the chromosome. Non-functional gene-fragments are common and a high proportion of the sequences are repeated elsewhere in the genome. This sequence arrangement suggests that the ends of chromosomes play a structural rather than a coding role and may be analogous to the highly repeated heterochromatic DNA of higher organisms. In order to evaluate the function of the ends of S. cerevisiae chromosomes, the rightmost 54-kb of DNA from chromosome I was investigated. The region contains 16 open reading frames (ORFs) and two tRNA genes. Gene-disruption studies indicated that none of these genes are essential for growth on rich or minimal medium, mating or sporulation.
In contrast to the central region where 80% of the genes are transcribed when cells are grown on rich medium, only seven ORFs and the two tRNA genes appeared to produce transcripts. Six of the transcribed ORFs were from the centromere-proximal part of the region, leaving the rightmost 35-kb with only a single sequence that is transcribed during vegetative growth. Two genes located 3 and 10-kb from the chromosome I telomere are almost identical to two genes located somewhat further from the chromosome VIII telomere. Surprisingly, the chromosome VIII copies were transcribed while the chromosome I genes were not. These results suggest that the chromosome I genes may be repressed by a natural telomere position effect. The low level of transcription, absence of essential genes as well as the repetitive nature of these sequences are consistent with their having a structural role in chromosome function.