Analyses of POL30 (PCNA) reveal positional effects in transient repression or bi-modal active/silent state at the sub-telomeres of S. cerevisiae.

Background: Classical studies on position effect variegation in Drosophila have demonstrated the existence of bi-modal Active/Silent state of the genes juxtaposed to heterochromatin. Later studies with irreversible methods for the detection of gene repression have revealed a similar phenomenon at the telomeres of Saccharomyces cerevisiae and other species. In this study, we used dual reporter constructs and a combination of reversible and non-reversible methods to present evidence for the different roles of PCNA and histone chaperones in the stability and the propagation of repressed states at the sub-telomeres of S. cerevisiae.

Results: We show position dependent transient repression or bi-modal expression of reporter genes at the VIIL sub-telomere. We also show that mutations in the replicative clamp POL30 (PCNA) or the deletion of the histone chaperone CAF1 or the RRM3 helicase lead to transient de-repression, while the deletion of the histone chaperone ASF1 causes a shift from transient de-repression to a bi-modal state of repression. We analyze the physical interaction of CAF1 and RRM3 with PCNA and discuss the implications of these findings for our understanding of the stability and transmission of the epigenetic state of the genes.

Conclusions: There are distinct modes of gene silencing, bi-modal and transient, at the sub-telomeres of S. cerevisiae. We characterise the roles of CAF1, RRM3 and ASF1 in these modes of gene repression. We suggest that the interpretations of past and future studies should consider the existence of the dissimilar states of gene silencing.