Effects of high mobility group proteins 1 and 2 on initiation and elongation of specific transcription by RNA polymerase II in vitro.

High mobility group proteins 1 and 2 (HMGs 1 and 2) are abundant chromosomal proteins of higher eukaryotes, which have been found to be enriched in regions of active chromatin. We have previously demonstrated that they can stimulate specific transcription in vitro by RNA polymerases II and III and overcome inhibition caused by added histones. Here we study whether these effects are mediated at the level of initiation or elongation of transcription. Additions of HMGs 1 and 2 and/or histones were found to have only small or no effect on the efficiency of elongation; this was determined by comparing the relative synthesis of transcripts of different lengths, ranging from 95 to 1535 bases. The observed stimulation cannot be explained by an increased utilization of initiation complexes for multiple rounds of transcription as a similar level of stimulation by HMGs 1 and 2 was seen when RNA synthesis was limited to one round per template DNA by addition of a low level of Sarkosyl after formation of initiation complexes. The effects of HMGs 1 and 2 were principally seen on the rate of formation of effective initiation complexes. These data are consistent with the hypothesis that HMGs 1 and 2 stimulate transcription by facilitating the formation of active initiation complexes on template DNA.