A multitude of heat shock transcription factors (HSFs) have been isolated and characterized from various plant species (17-23). Based on a phylogeny analysis of the DNA binding domains and organization of oligomerization domains, they have been assigned to class A and B of the plant HSF family (20,24 and this paper). None of the tested soybean or Arabidopsis HSF class B members were able to function as transcriptional activators and are, therefore, considered to be inert (26,59). Conversely, class A HSFs from tomato and Arabidopsis displayed an intrinsic transcriptional activation potential (26,50). There seems to be variation among plant class A HSFs regarding their transcriptional activation functions: some play a key role in activation of the heat shock response, while others act in an auxiliary capacity as HSF activity boosters (54). In contrast, the class B inert HSFs are able to trans-attenuate the transcriptional activity of activator HSFs (26). We postulated that heat shock regulation in plants may differ from metazoans by partitioning negative and positive functional domains onto separate HSF proteins (59). In plants two classes of HSFs exist: class A members which function as activators of HSP gene expression, and a novel class B (inert HSFs) which is largely specialized for repression, or attenuation, of the heat shock response.
Post-translational modifications of histone tails have dramatic ramifications on a variety of vital cellular functions. Removal of the acetyl groups from lysine residues is catalyzed by histone deacetylases (HDs). Many HDs are known to be components of multiprotein complexes such as SIN3 and NuRD that are involved in chromatin condensation and gene regulation. Plants contain a highly elaborated set of HDs with four distinct classes of these enzymes. Plant HDs have been implicated to play roles in transgene silencing, rDNA regulation, gene expression, and many developmental processes. Seventeen Arabidopsis HDs are apparent in Genbank as are numerous putative HD-interacting partners. Maize HDs have been extensively characterized biochemically and the use of powerful genetic tools currently available in Arabidopsis is rapidly accelerating the base of knowledge on the control circuitry of plant chromatin.
A plethora of heat shock transcription factors (HSFs) has been obtained from various plant species (33,45-48,50,51). The Arabidopsis genome sequencing project provided confirmation of the existence of at least twenty one HSFs which were classified into three major classes, A, B and C, and numerous subclasses (9). Members of HSF class A displayed differential transcriptional activities in tobacco protoplasts that varied from 15- to 50-fold above the control level. This diversity of activity levels may reflect HSF variations regarding their transcriptional activation functions- some of the members might be the major heat inducible HSFs (class A1 HSFs), while others act in an auxiliary capacity as HSF activity boosters (38). Two new class B HSFs showed no transcriptional activation potential; however, they differed significantly in their ability to bind to heat shock elements (HSEs). The efficiency in HSE binding was linked directly with the ability to suppress the activity of endogenous tobacco HSFs. The suppression of endogenous HSFs by class B members provides further evidence that class B HSFs are not transcriptional activators, but are able to trans-attenuate the transcriptional activity of bona fide activator HSFs (34,41). The transcriptional competency of class C HSFs has not been determined.
JavaScript is turned off in your web browser. Turn it on to take full advantage of this site, then refresh the page.