REPROGRAMMING OF NUCLEOLAR GENE-EXPRESSION DURING THE ACCLIMATIZATIONOP THE CARP

During seasonal acclimatization of eurythermal fish, the nucleolus of the hepatocyte undergoes ultrastructural reprogramming. In winter accl imatized carp, the nucleolar components are segregated, a condition th at suggests a decreased transcription of rRNA. The same nucleolar reor ganization was observed when pituitary cells from winter- and summer-a cclimatized carp were examined. In situ analyses of nucleolar RNA reve aled a marked lowering of RNA content in the segregated nucleoli. Acco rdingly, in vitro synthesis of RNA was shown to be significantly lower in pituitary tissue from cold-acclimatized fish where precursor accum ulated. Conversely, in pituitary tissue from summer-adapted fish the r ate and extent of synthesis and of rRNA processing was notably higher. The involvement of pre-rRNA processing events during seasonal acclima tization was corroborated by the strong differences of U3 RNA content detected by in situ hybridization in pituitary cells from summer- and winter-fish. When RNA polymerase I activity from both acclimatized sta tes were assayed, no differences were detected. Thus, it appears that in fish RNA polymerase I itself does not play an important role in the control of nucleolar gene expression and the nucleolar gene expressio n reprograming that the seasonal rearrangement represents might involv e, among the many nucleolar-specific proteins, transcription factors.