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RNA polymerase I (Pol I) transcription in the yeastSaccharomyces cerevisiae is greatly stimulatedin vivo andin vitro by the multiprotein complex, upstream activation factor (UAF). UAF binds tightly to the upstream element of the rDNA promoter, such that once bound (in vitro ), UAF does not readily exchange onto a competing template. Of the polypeptides previously identified in purified UAF, three are encoded by genes required for Pol I transcriptionin vivo :RRN5 ,RRN9 , andRRN10 . Two others, p30 and p18, have remained uncharacterized. We report here that the N-terminal amino acid sequence, its mobility in gel electrophoresis, and the immunoreactivity of p18 shows that it is histone H3. In addition, histone H4 was found in UAF, and myc-tagged histone H4 could be used to affinity-purify UAF. Histones H2A and H2B were not detectable in UAF. These results suggest that histones H3 and H4 probably account for the strong binding of UAF to DNA and may offer a means by which general nuclear regulatory signals could be transmitted to Pol I.

Histones H3 and H4 are components of upstream activation factor required for the high-level transcription of yeast rDNA by RNA polymerase I