Ovary-specific expression of a gene encoding a divergent α-tubulin isotype in Xenopus

We are investigating the structure and regulation of α-tubulin genes expressed in amphibian oocytes. We have characterised here a gene, XαT207, that produces a major α-tubulin mRNA of Xenopus laevis ovary. XαT207 mRNA was not detected in other frog tissues and its production may therefore be a key identifying feature of ovarian differentiation. In comparison to the tubulin isotypes so far described in mammals and Xenopus, the α-tubulin encoded by XαT207 is divergent in overall amino acid sequence, particularly in the N-terminal region between residues 39–50. This pattern of divergence is also displayed by the ovary-specific α-tubulin gene of Drosophila, Dα4, although the two genes do not appear to be orthologous. The development of specialised microtubular structures and activities in oocytes, eggs and early embryos may then be correlated with the expression of a divergent α-tubulin isotype in a wide range of organisms. To understand the basis of the ovary-specific expression of XαT207 we examined the transcriptional activity of wild type and mutant promoters after their microinjection in Xenopus oocytes. Only 65 bp upstream of the initiation site were required for full activity of the XαT207 promoter, and an element fitting the Y-box consensus was involved in controlling the efficiency of initiation. Previous oocyte injection experiments have implicated the Y-box in the oocyte-specific transcription of genes that are also expressed in other cell types, so its involvement in the oocyte-restricted expression of XαT207 further suggests that transcription factors recognising the Y-box normally regulate gene expression during oocyte development. Since a Y-box also occurs in the Dα4 promoter, our results suggest that in both organisms oocyte-specific expression of a divergent α-tubulin could be achieved by a common mechanism.