Two isoforms of FTZ-F1 messenger RNA: molecular cloning and their expression in the frog testis

FTZ-F1, a member of the orphan nuclear receptors, is a transcriptional factor regulating the expression of the fushi tarazu gene in Drosophila (Lavorgna et al., 1991. Science 252, 848-851). Previously, we cloned a frog homologue of FTZ-F1 (rrFTZ-F1alpha; GenBank Accession No. AB035498). In this study, we isolated the rrFTZ-F1beta cDNA encoding a protein of 469 amino acids. Then, expressions of two types (alpha and beta) of rrFTZ-F1 mRNAs were examined during development of embryos and gonads in the frog Rana rugosa. They were expressed in the embryo at stage 12. Expressions of both the alpha and beta mRNAs became stronger in the testis of frogs at stage XXV and were most prominent in that of frogs 2months after metamorphosis. In the former testis, spermatogonia were the only germ cells in the seminiferous tubules, whilst postmeiotic cells were observed in the latter testis. Expression of the typealpha mRNA was more prominent. In addition, we cloned the regions with either exon I or II of the rrFTZ-F1 gene. Genomic structure analysis revealed that rrFTZ-F1beta is a partial exon I-truncated variant of rrFTZ-F1alpha. The results suggest that rrFTZ-F1alpha and -beta are expressed from the same gene by alternative splicing and that they may play an important role(s) in differentiation of premeiotic germ cells in the testis of the frog R. rugosa.