Roles of ABF1, NPL3, and YCL54 in silencing in Saccharomyces cerevisiae.

A sensitized genetic screen was carried out to identify essential genes involved in silencing in Saccharomyces cerevisiae. This screen identified temperature-sensitive alleles of ORC2 and ORC5, as described elsewhere, and ABF1, NPL3, and YCL54, as described here. Alleles of ABF1 that caused silencing defects provided the genetic proof of Abflp's role in silencing. The roles of Npl3p and Ycl54p are less clear. These proteins did not act exclusively through any one of the three protein binding sites of the HMR-E silencer. Unlike the orc2, orc5, and abf1 mutations that were isolated in the same (or a similar) screen for silencing mutants, neither temperature-sensitive mutation in NPL3 or YCL54 caused overt replication defects.