A MULTIPLICITY OF MEDIATORS - ALTERNATIVE FORMS OF TRANSCRIPTION COMPLEXES COMMUNICATE WITH TRANSCRIPTIONAL REGULATORS

The already complex process of transcription by RNA polymerase lr has become even more complicated in the last few years with the identifica tion of auxiliary factors in addition to the essential general initiat ion factors, In many cases these factors, which have been termed media tors or co-activators, are only required far activated or repressed tr anscription, In some cases the effects are specific for certain activa tors and repressors, Recently some of these auxiliary factors have bee n found in large complexes with either TBP, as TBP-associated factors (TAFs) in the general factor TFIID, or with pal 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 descri bed, including one that contains a tissue-specific TAF and another wit h a cell type-specific form of TBP, In addition, in yeast there are at least two forms of the 'holoenzyme' distinguished by their mediator c omposition and by the spectrum of transcripts whose expression they af fect, Genetic and biochemical analyses have begun to identify the inte ractions between the components of these complexes and the ever increa sing family of DNA binding regulatory factors, These studies are compl icated 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.