Reprogramming of nucleolar gene expression during the acclimatization of the carp

During seasonal acclimatization of eurythermal fish, the nucleolus of the hepatocyte undergoes ultrastructural reprogramming. In winter acclimatized carp, the nucleolar components are segregated, a condition that suggests a decreased transcription of rRNA. The same nucleolar reorganization was observed when pituitary cells from winter- and summer-acclimatized carp were examined. In situ analyses of nucleolar RNA revealed a marked lowering of RNA content in the segregated nucleoli. Accordingly, in vitro synthesis of RNA was shown to be significantly lower in pituitary tissue from cold-acclimatized fish where precursor accumulated. Conversely, in pituitary tissue from summer-adapted fish the rate and extent of synthesis and of rRNA processing was notably higher. The involvement of pre-rRNA processing events during seasonal acclimatization 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 states 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 expression reprogramming that the seasonal rearrangement represents might involve, among the many nucleolar-specific proteins, transcription factors.