Evolution of Sir1p-ORC interactions and modulation of ORC's silencing
and replication functions in yeast heterochromatin.
Kristopher H. McConnell, Catherine A. Fox
Biomolecular
Chemistry, U.W. Madison, 1300 University Ave, Madison, WI 53706-1532,
United States
We are investigating the mechanisms that balance ORC's
replication and silencing functions at HMR. Previous work
indicated that HMR contained a number of regions that could
provide for origin function on the chromosome. One of these regions, the
telomere-proximal region flanking HMR-E, functions as both an
ORC-dependent chromosomal replication origin and a potent anti-silencer
in strains that lacked the HMR-E silencer ACS. We are performing
a number of experiments to learn how a telomere-proximal origin inhibits
silencing. In one set of experiments we have shown that over-expression
of Sir1p can enhance silencing in strains with a defective HMR-E
silencer only if this telomere-proximal origin is deleted. This Sir1p-dependent silencing requires the Orc1p N-terminus regardless of whether
the HMR-E silencer contains an intact ACS. We postulate that the
telomere-proximal anti-silencer origin prevents Sir1p from binding to
HMR by an as yet unknown mechanism, but that Sir1p-ORC
interactions are critical for Sir1p function at HMR even in the
absence of the bona fide silencer ACS. In a second series of experiments
we are testing the conservation of a Sir1p-ORC interaction by
identifying SIR1 homologues in closely related yeast species.
Return to YGM 2002 Home at SGD