Developmental regulation of a silkworm gene encoding multiple GATA-type transcription factors by alternative splicing.

Gene BmGATA beta of the silkworm Bombyx mori was previously shown to encode factor BCFI, which regulates the expression of a class of chorion genes expressed during the late stages of choriogenesis. We now show that the expression of the BmGATA beta gene is spatially and temporally regulated by alternative splicing that generates two major (BmGATA beta 1 and BmGATA beta 2) and one minor (BmGATA beta 3) mRNA isoforms of non-identical tissue distribution. The three isoforms differ in the organization of the DNA-binding domains of the corresponding polypeptides. While all three isoforms are expressed in ovarian follicular cells and in testes, only one of them, BmGATA beta 1, is gonad-specific. BmGATA beta 2 is expressed in a variety of other larval and pupal tissues, while BmGATA beta 3 is detected in some pupal but none of the larval tissues. Analysis of RNA isolated from follicular cells of developing ovarian follicles has shown that the onset of ovarian transcription for all three mRNA isoforms occurs during late vitellogenesis, and that the level of accumulated mRNA declines significantly at the onset of choriogenesis. Coincident with the onset of late chorion gene expression, we have observed a significant change in the preference of splice site selection in favour of the one that results in the generation of BmGATA beta 1 mRNA. The transcriptional activation of the BmGATA beta gene in follicular cells during late vitellogenesis correlates with the previously demonstrated initial accumulation of factor BCFI in the cytoplasm of follicular cells and its appearance in follicular cell nuclei only during the late stages of choriogenesis. The relationship between factor BCFI and the different polypeptides encoded by the three BmGATA beta mRNA isoforms is discussed.