Biologjia Online

A few discrete portions of the Saccharomyces cerevisiae genome, such as the chromosome ends within 5–10 kilobase pairs (kb) of telomeres and the silent mating-type regions named HMR and HML, are defined as transcriptionally silent because they cause repression of an RNA polymerase II gene placed within them (reviewed comprehensively in Ref. 1). S. cerevisiae silencing is a form of transcriptional repression that is long range, occurring over distances larger than a single gene. Silencing occurs in only certain chromosomal regions and depends more on a gene's position in the genome than its promoter sequence. Another defining characteristic is a stable inheritance pattern during cell division, indicating that the repressive state is duplicated along with the underlying DNA sequences during DNA replication. These features make S. cerevisiae transcriptional silencing functionally akin to position-effect-variegation in Drosophila melanogaster and X-chromosome inactivation in female mammals