PAF49 Acetylation Affects the Assembly of the PAF49/PAF53 Heterodimer with Pol I

Abstract

Mammalian PAF49 and PAF53, and their yeast homologues form heterodimers play important roles in transcription. While the yeast homologues are “not essential” proteins, yeast cells deficient in rpa49, the homologue of PAF53, grow very slowly compared to wild-type cells. We have found that knockdown of PAF53 with either an antisense RNA or an siRNA results in the inhibition of rDNA transcription in vivo demonstrating its importance in rDNA transcription. We and others have reported that the association of PAF49/PAF53 with Pol I is regulated. However, the mechanism that regulates this association is unknown. We have recently found that PAF49 is acetylated on multiple sites. The acetylation of PAF49 does not affect heterodimerization, but it does affect the interaction of the heterodimer with Pol I. Hypoacetylation increases the affinity to Pol I. We have found that the heterodimer interacts with both SirT7 and Rrn3. SirT7 is a nucleolar deacetylase associated with elevated rates of rDNA transcription. However, its role in rDNA transcription is not known. We propose a biochemical interaction between the Pol I-associated heterodimer and Rrn3 and that this interaction facilitates the recruitment of Rrn3 to the polymerase. Interestingly, the acetylation state of the heterodimer affects the interaction with Rrn3 differently than assembly with Pol I. As the recruitment of Rrn3 to Pol I is essential to transcription initiation in yeast and mammals, our results provide a greater understanding of the regulation of Rrn3 function and provide biochemical underpinning for the roles of PAF49/PAF53 in transcription.