OVERLAPPING TRANSCRIPTION BY RNA POLYMERASE-II AND POLYMERASE-III OF THE XENOPUS TFIIIA GENE IN SOMATIC-CELLS

Xenopus TFIIIA gene expression is developmentally regulated. The per c ell level of steady-state TFIIIA mRNA present in oocytes is drasticall y reduced (10(6)-fold) in adult somatic cells, suggesting regulation a t the transcriptional level. This is supported by the presence of TFII IAmRNAs with different 5'-ends in oocytes and in somatic cells, which suggests the presence of distinct cell-/stage-specific promoters, i.e. a strong oocyte-specific and a weak somatic-specific promoter. Here, by mapping the 5'-ends of TFIIIA RNAs found in Xenopus somatic cells, we document the activity not only of an upstream somatic-specific prom oter, but also of a downstream ''oocyte-specific'' promoter that gives rise to a minor population of somatic TFIIIA mRNAs initiated from the same transcription start site utilized in oocytes. This result explai ns the presence of an oocyte-type form of TFIIIA protein in somatic ce lls. Furthermore, by characterizing transcription from the Xenopus TFI IIA promoter in somatic cell nuclear extracts and comparing the in vit ro transcription start sites with the 5'-ends of endogenous somatic TF IIIA transcripts in poly(A)(+) and poly(A)(-) RNA fractions, we show t hat RNA polymerase III initiates at position -70 of the ''oocyte-speci fic'' core promoter and transcribes through the promoter and into the TFIIIA coding sequences in somatic cells. These results suggest a poss ible polymerase III transcription-mediated down-regulation of the oocy te-specific TFIIIA promoter in Xenopus somatic cells.