Transcriptional silencing at the silent mating type loci and telomeres
requires the dedicated SIR proteins and some general purpose
proteins with silencing roles. Among these is the six subunit Origin
Recognition Complex, which also is required for replication initiation.
We found that one subunit is required both at the G1/S transistion and
for completion of mitosis. Genetic data suggest that the mitotic
function of ORC may involve sister chromatid separation. In addition, we
found that chromosomes with defective ORC complexes inhibit cell cycle
progression in a dominant manner. In order to gain a better
understanding of ORC's essential functions, extragenic suppressors of
the temperature sensitivity of an orc5-1 mutant strain were
isolated, which may identify new genes involved in silencing. The
YCL54 gene encodes another protein required for silencing. A
ycl54 mutant strain shows compromised silencing at the mating-type loci and telomeres, but improved silencing of a MET15 marker
gene present in rDNA. Interestingly, the ycl54 mutant also has
defects in rRNA transcription. SIR2 is an evolutionary conserved
molecule, with homologs in mammals, eubacteria and archeabacteria,
however the function of these homologs remains unknown. A null allele of
a SIR2 homolog in Drosophila melanogaster was generated
and a phenotypic characterization of the effects of this null allele
will be presented.
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