Figure 8.—

Model for SIR2 transcriptional regulation. The nucleolar rDNA copies are presumed to be a part of a global silencing “sensor” that signals to control SIR2 expression. This signal is capable of integrating information on the total number of rDNA copies (Michel et al. 2005) as well as the overall strength of rDNA silencing as in the R95A mutant. The wavy lines might represent the Pol II transcripts described by Kobayashi and Ganley (2005) and Ide et al. (2010) to play a role in rDNA copy number control or might be some other signaling molecule; the dashed lines imply that there may be intermediate steps in the regulatory paths.