Yeast is unusual in that its transcriptional activators cannot
work over long distances. But in the 4 January Nature, de Bruin
et al. report that the looping of heterochromatic-like telomere
regions corrects this shortcoming (Nature 2001, 409:109-113).
They place a binding site for the activator Gal4 downstream of a
reporter gene. The site is inactive when the reporter is at an
internal chromosomal locus, but active when the gene cassette is
placed, in either orientation, near the telomere. Only at the
telomere is Gal4 associated with promoter DNA, as assayed by
chromatin immunoprecipitation. This leads de Bruin et al. to
suggest that telomere looping brings the activator close to the
promoter to activate transcription. Such looping may be
impossible elsewhere in the yeast genome because yeast lacks
either the heterochromatic regions or auxiliary looping proteins
present in other species.