Mp. Chang et Ja. Jaehning, A MULTIPLICITY OF MEDIATORS - ALTERNATIVE FORMS OF TRANSCRIPTION COMPLEXES COMMUNICATE WITH TRANSCRIPTIONAL REGULATORS, Nucleic acids research, 25(24), 1997, pp. 4861-4865
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.