Substances in nuclear wheat germ extracts which interfere with polymerase III transcriptional activity in vitro.

Wheat germ nuclear extracts inhibited an active yeast polymerase III (pol III) transcription extract. We isolated two chromatin-associated fractions which harbored biochemically distinguishable inhibitory activities, each contributing about 40-50% to the total inhibitory activity. One fraction, which was released from the chromatin upon treatment with 350 to 900 mM NaCl, was purified to homogeneity and identified as histone H1. It inhibited the yeast extract by excluding the transcription machinery from the template DNA. It can be partially antagonized by additional nontemplate DNA together with templates that have strong pol III promoters. The other fraction, which was released from the chromatin between 0 and 350 mM NaCl, inhibited transcription by affecting transcription complex formation partially through transcription factor-inhibitor interactions. Furthermore, it affected the rate of transcription reinitiation but not the elongation rate. Ways to move towards an active DNA-dependent pol III plant extract are discussed.