Novel critical role of a human Mediator complex for basal RNA polymerase II transcription

Human Mediator complexes have been described as important bridging factors that enhance the effect of activators in purified systems and in chromatin. Here we report a novel basal function of a human Mediator complex. A monoc lonal antibody was generated that depleted the majority of Mediator compone nts from crude cell extracts. The removal of human Mediator abolished trans cription by RNA polymerase II. This was observed on all genes tested, on TA TA-containing and TATA-less promoters, both in the presence and absence of activators. To identify the relevant complex a combined biochemical and imm unopurification protocol was applied. Two variants termed Mediator and basa l Mediator were functionally and structurally distinguished. Basal Mediator function relies on additional constraints, which is reflected in the obser vation that it is essential in crude but not in purified systems. We conclu de that basal Mediator is a novel general transcription factor of RNA polym erase II.