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

We are investigating the structure and regulation of alpha-tubulin genes expressed in amphibian oocytes. We have characterised here a gene, X alpha T207, that produces a major alpha-tubulin mRNA of Xenopus laevis ovary. X alpha T207 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 alpha-tubulin encoded by X alpha T207 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 alpha-tubulin gene of Drosophila, D alpha 4, 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 alpha-tubulin isotype in a wide range of organisms. To understand the basis of the ovary-specific expression of X alpha T207 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 X alpha T207 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 X alpha T207 further suggests that transcription factors recognising the Y-box normally regulate gene expression during oocyte development. Since a Y-box also occurs in the D alpha 4 promoter, our results suggest that in both organisms oocyte-specific expression of a divergent alpha-tubulin could be achieved by a common mechanism.