DISTINCT STRESS-INDUCIBLE AND DEVELOPMENTALLY-REGULATED HEAT-SHOCK TRANSCRIPTION FACTORS IN XENOPUS OOCYTES

The presence of a maternal pool of heat shock factor (HSF) in Xenopus oocytes has been suggested by two lines of evidence from previous stud ies. First, heat shock response element (HSE)-binding activity is indu ced in heat-shocked eggs and embryos prior to expression of zygotic HS F. Second, expression from microinjected heat shock protein promoters in oocytes is induced upon heat shock. To date, however, endogenous oo cyte HSF molecules have not been detected, nor has induction of HSE-bi nding activity been directly demonstrated. Here we report the detectio n of distinct stress-inducible and developmentally regulated HSE-bindi ng activities of endogenous oocyte factors. Exposure of defolliculated oocytes to heat, cadmium, and arsenite resulted in the formation of a n HSE-specific complex detectable by gel mobility shift assay. Inducti on of HSE-binding activity by each of these stressors corresponded to increased expression from a microinjected hsp70 promoter. The stress-i nducible HSE-binding complex was recognized by antiserum against mamma lian HSF1, but not by HSF2 antiserum, suggesting that a Xenopus homolo gue of HSF1 is the major component of this activity. The HSE-binding a ctivity of HSF1 was induced by stress treatments of stage I through VI oocytes, an indication that it is responsive to stress throughout oog enesis. During recovery from heat shock, the HSF1-HSE complex rapidly declined to control levels, but was induced for prolonged periods in o ocytes exposed to continuous stress, a pattern unlike the transient ac tivation previously observed in fertilized eggs or embryos. The kineti cs of HSF1 activation in oocytes suggests that a key protein(s) regula ting attenuation of the stress response is present at exceedingly low levels or is somehow modified during preembryonic development. We also detected an unusual constitutive HSE-binding complex in unstressed st age I and II oocytes, but not in later stage oocytes, eggs, developing embryos, or A6 cells. This constitutive complex was unaffected by hea t or chemical treatments and was not recognized by either HSF1 or HSF2 antiserum. Appearance of the constitutive HSE-binding activity during oogenesis corresponded closely with peak levels of hsp70 mRNA detecte d by Northern blot analysis of RNA from staged oocytes. We suggest tha t the constitutive HSE-binding activity in early oocytes is formed by a unique developmentally regulated heat shock factor that may play a r ole in the expression of heat shock proteins during early stages of oo genesis. (C) 1997 Academic Press