Cells have evolved multiple strategies to adapt the composItIon and
quality of their protein equipment to needs imposed by changing
conditions within the organism. Extracellular stimuli that inform cells
about such needs are hormones, cytokines and neurotransmitters, which
bind to specific cell surface receptors. Inside the cell, secondary
signals are then produced which, ultimately, initiate the expression of
proteins giving novel functional properties to the stimulated cells.
This process can be controlled at a transcriptional,
posttranscriptional, translational or posttranslational level. Extensive
research over the past fifteen years has shown that transcriptional
regulation is probably the most impor- tant strategy used to control the
production of new proteins in response to hormonal signals. At the level
of gene transcription, the initiation of mRNA synthesis is most
frequently used to govern gene expression. The key elements controlling
transcription initiation in eukaryotes are acti- vator proteins
(transactivators) that bind in a sequence-specific manner to short DNA
sequences in the proximity of genes. The activator binding sites are
elements oflarger control units, called promoters and enhancers, which
bind many distinct proteins that may synergize or negatively cooperative
with the activators. The de novo binding of an activator to DNA or, if
already bound to DNA, its functional activation is what ultimately turns
on a high-level expression of genes. In this second volume of Inducible
Gene Expression, leading scientists in the field review eight eukaryotic
transactivators that allow cells to respond to hormonal stimuli by the
expression of new proteins.
