We have
described an alternative form of the RNA polymerase II holoenzyme
lacking the Srbps, but containing Paf1p, Hpr1p, Ccr4p and Cdc73p. We
have shown that the Paf1p complex is important for the cell wall
biosynthetic pathway controlled by the Pkc1p/MAP kinase cascade. Many
of these genes are also regulated during the cell cycle. In addition,
using differential display and microarray analysis, we have identified
many cell cycle regulated genes whose expression is reduced in
paf1 strains. To ask if the Paf1p complex is important for
cyclic expression of these genes we used alpha factor synchronized
cells to assay transcript abundance from CLN1, RNR1,
FAR1, CYB5, and FKS1. We found that although
transcript levels are reduced 2 to 5 fold throughout the cell cycle,
the amplitude of cycling is unchanged in paf1 relative to
wild-type. Swi4p and Mbp1p are DNA binding factors with overlapping
functions in the control of genes in both cell wall maintenance and
cell cycle. A swi4 mbp1 double mutant is lethal demonstrating
that these overlapping functions are essential. We therefore analyzed
genetic interactions between paf1, swi4, and
mbp1. We found that paf1 swi4 is synthetically lethal,
but paf1 mbp1 does not have an enhanced phenotype. These
results suggest that Paf1p is required for Mbp1p function. We are
exploring this hypothesis using promoter/reporter constructs in mutant
backgrounds to elucidate the role of the Paf1p complex in the
expression of cell cycle regulated genes.
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