Regulation of the Xenopus laevis transcription factor IIIA gene during oogenesis and early embryogenesis: Negative elements repress the O-TFIIIA promoter in embryonic cells

Expression of the Xenopus laevis transcription factor IIIA (TFIIIA) gene is developmentally regulated. In this study we have used defined nucleotide mutations to map cis-elements involved in transcriptional regulation of the promoter for oocyte-TFIIIA (O-TFIIIA) in stage II–IV oocytes, stage VI oocytes, and tail bud embryos. During oogenesis O-TFIIIA mRNA levels decline 5- to 10-fold, and during early embryogenesis O-TFIIIA mRNA levels decline approximately 10 6-fold per cell. In stage II–IV oocytes we find evidence for at least three distinct positive-acting cis-elements that contribute to the efficient expression of O-TFIIIA. These elements are located between −1800 to −425, −280 to −235, and −235 to −220. The most distal cis-element(s) appears to be developmentally regulated during oogenesis, since deletion of nucleotide sequences from −1800 to −425 does not reduce O-TFIIIA expression in stage VI oocytes. However, the two cis-elements located between −280 to −235 and −235 to −220 are required for the efficient expression of O-TFIIIA in stage VI oocytes. In tail bud embryos we find evidence for several developmentally regulated positive and negative cis-elements involved in O-TFIIIA expression. The positive-acting cis-elements are located between −159 to −110 and −110 to −58. The negative-acting cis-elements are found at positions −425 to −350 and −200 to −159. In addition to the developmentally regulated elements controlling O-TFIIIA gene expression in tail bud embryos, the positive-acting cis-elements active during oogenesis (located between −280 to −235 and −235 to −220) are also active during early embryogenesis. Thus, transcription from the O-TFIIIA promoter appears to be regulated by a combination of constitutive positive factors and developmentally regulated positive and negative factors during oogenesis and early embryogenesis.