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.