A unique role for G1 cyclins in transcription by RNA polymerase
III.
Neelam Desai, Ian Willis
Department of
Biochemistry, Albert Einstein Col. Medicine, 1300 Morris Park Ave,
Bronx, NY 10461, USA
Pol I and pol III gene transcription in
metazoans is repressed in mitosis, increases in early G1 phase but only
becomes maximally activated after the cells are committed to a round of
division. In Saccharomyces cerevisiae, there is no global
repression of transcription during mitosis and changes in transcription
by pols I or pol III at other stages in the cell cycle have not been
reported. In this work, we have examined tRNA gene transcription in
yeast cells synchronized using different regimes and find that the
levels of short-lived precursor tRNAs fluctuate in a manner consistent
with the need to reassemble preinitiation complexes after DNA
replication. During these studies we found that high levels of pol III
transcription in G1 phase are dependent on G1 cyclins (Clns). G1 cyclin
deprivation, leading to cell cycle arrest at START, produced a sharp
drop in tRNA and 5S rRNA synthesis that was not seen following treatment
with alpha factor, in conditional cdc28 strains or in cells
arrested in mitosis with nocodazole. The functional link between G1
cyclins and pol III transcription was shown to be independent of Cdc28
kinase activity and did not appear to require a physical association
between the Clns and Cdc28. We propose that the elevation of G1 cyclin
levels at START serves to activate pol III gene transcription to ensure
continued cell growth during the division cycle.
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