Overlapping transcription by RNA polymerases II and III of the Xenopus TFIIIA gene in somatic cells.

Xenopus TFIIIA gene expression is developmentally regulated. The per cell level of steady-state TFIIIA mRNA present in oocytes is drastically reduced (10(6)-fold) in adult somatic cells, suggesting regulation at the transcriptional level. This is supported by the presence of TFIIIA mRNAs 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 promoter, but also of a down-stream "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 explains the presence of an oocyte-type form of TFIIIA protein in somatic cells. Furthermore, by characterizing transcription from the Xenopus TFIIIA promoter in somatic cell nuclear extracts and comparing the in vitro transcription start sites with the 5'-ends of endogenous somatic TFIIIA transcripts in poly(A)+ and poly(A)- RNA fractions, we show that RNA polymerase III initiates at position -70 of the "oocyte-specific" core promoter and transcribes through the promoter and into the TFIIIA coding sequences in somatic cells. These results suggest a possible polymerase III transcription-mediated down-regulation of the oocyte-specific TFIIIA promoter in Xenopus somatic cells.