High mobility group proteins 1 and 2 stimulate transcription in vitro by RNA polymerases II and III.

We have used specific in vitro transcription as a functional assay to study the effects of the major chromosomal proteins, histones and high mobility group (HMG) proteins 1 and 2, on transcription by RNA polymerases II and III. HMG proteins 1 and 2 can stimulate transcription by both RNA polymerases II and III; maximal stimulation (up to 20-fold) is seen when HMG proteins 1 and 2 are mixed with template DNA prior to the addition of whole cell transcription lysate. HMG proteins 1 and 2 are also able to overcome inhibition of transcription caused by histones; maximal enhancement of transcription (at least 60-fold) is seen when histones and then HMG proteins 1 and 2 are added to DNA before addition of transcription lysate. This suggests that stimulation of transcription by HMG proteins 1 and 2 in the presence of histones is not due to a simple competition between histones and HMG proteins for binding to DNA. It appears likely that HMG proteins 1 and 2 stimulate transcription in a nonspecific manner, perhaps altering the template in such a way as to allow increased accessibility of transcription factors or rate of elongation for both RNA polymerases II and III.