HMG17 IS A CHROMATIN-SPECIFIC TRANSCRIPTIONAL COACTIVATOR THAT INCREASES THE EFFICIENCY OF TRANSCRIPTION INITIATION

We have examined the effect of HMG17 on transcription by RNA polymeras e II by the assembly and analysis of HMG17-containing chromatin templa tes consisting of regularly spaced nucleosomal arrays. Structural anal ysis of the chromatin indicated that HMG17 is incorporated into chroma tin in a physiological manner with the full complement of core histone s. The transcriptional studies revealed that HMG17 stimulates transcri ption in conjunction with the sequence-specific activator GAL4-VP16. T his effect was observed with chromatin, but not with non-nucleosomal t emplates, and required the presence of HMG17 during chromatin assembly . The incorporation of HIMG17 into chromatin resulted in a 7- to 40-fo ld stimulation of GAL4-VP16-activated transcription to levels that wer e comparable to those observed with histone-free DNA templates. In con trast, transcription from HMG17-eontaining chromatin was not detectabl e in the absence of GAL4-VP16 or with a GAL4 derivative [GAL4(1-147)] lacking the VP16 activation domain. Finally, the incorporation of HMG1 7 into chromatin was found to increase the efficiency of transcription initiation, but not the extent of transcriptional elongation. Thus, H MG17 is a chromatin-specific transcriptional coactivator that increase s the efficiency of initiation of transcription by RNA polymerase II.