The chromosome ends of Saccharomyces cerevisiae are complex mosaics of several different types of telomeric and subtelomeric elements known as X element core sequences, X element combinatorial repeats, telomeric repeats, and Y' elements (2). The X element core sequence, a small conserved element of ~475 bp containing an ARS sequence and in most cases an Abf1p binding site, is the only region shared by all chromosome ends (2, 1). The X element combinatorial repeats, formerly known as subtelomeric repeats or STRs, are located between the X element core sequence and the telomeric end and are usually present as a combination of one or more of several types of smaller elements designated D, C, B, or A (2, 1). The telomeric repeat is a G-rich terminal sequence of the form (TG_1-3)_n that is maintained by telomerase (1). The Y' element is a helicase-encoding repetitive sequence found in many but not all subtelomeric regions next to the telomeric repeats, or adjacent X element combinatorial repeats, either as a single copy or tandem repeat of two to four copies (3, 1, 4). Possible functions of telomeric regions include roles in chromosomal segregation, maintenance of chromosome stability, recombinational sequestering, or as a barrier to transcriptional silencing (2, 1).

TEL09L, the left telomeric region of chromosome IX, is composed of an X element core sequence TEL09L-XC containing an ARS consensus sequence and an Abf1p binding site, X element combinatorial repeats TEL09L-XR of the D type containing a group I mitochondrial bI4 intron insertion, a long Y' (Y'-L) element TEL09L-YP containing an ARS consensus sequence, a region of 36-bp repeats, one uncharacterized ORF YIL177C and one dubious ORF YIL177W-A, and a short terminal stretch of telomeric repeats TEL09L-TR.

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Further information can be found at the website of Dr. Ed Louis, Institute of Genetics, The University of Nottingham.

Last updated: 2003-10-29