HISTONES H3 AND H4 ARE COMPONENTS OF UPSTREAM ACTIVATION FACTOR REQUIRED FOR THE HIGH-LEVEL TRANSCRIPTION OF YEAST RDNA BY RNA-POLYMERASE-I

RNA polymerase I (Pol I) transcription in the yeast Saccharomyces cere visiae trine is greatly stimulated iir vivo and in vitro by the multip rotein complex, upstream activation factor (UAF). UAF binds tightly to the upstream element of the rDNA promoter, such that once bound (in v itro), 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 transcription in vivo: RRN5, RRN9, and RR N10. Two others, p30 and p18, have remained uncharacterized. We report here that the N-terminal amino acid sequence, its mobility in gel ele ctrophoresis, and the immunoreactivity of p18 shows that it is histone H3, In addition, histone H4 mas found is 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 prob ably account for the strong binding of UAF to DNA and may offer a mean s by which general nuclear regulatory signals could he transmitted to Pol I.