Regulation of the ribosomal RNA gene expression during the acclimatization of the carp cyprinus carpzo

During seasonal acclimatization the nucleolus of the hepatocyte and pituitary cells of the carp fish undergoes ultrastructural reprogramming. In winter, the nucleolar components are segregated which morphologically reflects the temporary inactivation of the ribosomal gene expression. As revealed by in situ analyses a marked lowering of RNA content occurs during this state, suggesting that cold adaptation indeed results in a reduction of rRNA transcription. In vitro synthesis of rRNA is significantly lower in pituitary tissue from coldacclimatized fish where pre-rRNA accumulates. Concurrently, strong differences of U3 RNA content were also detected by in situ hybridization in pituitary cells from summer- and winter- acclimatized fish which corroborates the involvement of pre-rRNA processing in the seasonal compensatory response. Strikingly, no differences were detected in RNA polymerase I (pol l) activity in both acclimatized conditions- Thus, it appears that in fish pol I itself does not play an important role in the control of rRNA expression and the reprogramming that the seasonal rearrangement represents might involve, among the many nuclear-specific proteins, transcription factors. In this context we have isolated clones containing carp 18S, 5.8S and 28S rRNA sequences and are currently investigating upstream elements located in the non- transcribed spacer. Comparisons of cis control elements with sequences from other species are beine examined. These sequences should allow the study of carp RNA pol I transcription factors.