A multiplicity of mediators: alternative forms of transcription complexes communicate with transcriptional regulators.

The already complex process of transcription by RNA polymerase II has become even more complicated in the last few years with the identification of auxiliary factors in addition to the essential general initiation factors. In many cases these factors, which have been termed mediators or co-activators, are only required for activated or repressed transcription. In some cases the effects are specific for certain activators and repressors. Recently some of these auxiliary factors have been found in large complexes with either TBP, as TBP-associated factors (TAFs) in the general factor TFIID, or with pol II and a subset of the general factors, referred to as the 'holoenzyme'. Although the exact composition of these huge assemblies is still a matter of some debate, it is becoming clear that the complexes themselves come in more than one form. In particular, at least four forms of TFIID have been described, including one that contains a tissue-specific TAF and another with a cell type-specific form of TBP. In addition, in yeast there are at least two forms of the 'holoenzyme' distinguished by their mediator composition and by the spectrum of transcripts whose expression they affect. Genetic and biochemical analyses have begun to identify the interactions between the components of these complexes and the ever increasing family of DNA binding regulatory factors. These studies are complicated by the fact that individual regulatory factors often appear to have redundant interactions with multiple mediators. The existence of these different forms of transcription complexes defines a new target for regulation of subsets of eukaryotic genes.