In addition, beta-galactosidase appears to persist beyond the time when the TAP transcript is no longer detectable by in situ hybridization. Alternatively, the lacZ fusion gene could be missing cis-elements necessary for turning off tap expression and therefore lacZ is still transcribed after the endogenous gene has been shut off. Since the lacZ gene used in the biparous promoter construct encodes a cytoplasmic protein, this result indicates that the N-terminal portion of Biparous is able to direct nuclear localization. In this region, repo is expressed in the segmental nerve glia, the intersegmental nerve glia, and the exit glia. Nevertheless, since repo is a glial-specific gene, the colocalization provides evidence that the tap gene is transcribed in precursor cells that generate glial progeny.
Some cells simultaneously express both proteins.
Both biparous and gcm are expressed relatively early during CNS development, with both transcription factors, turned on before many neuronal and glial progenitors have completed their final mitosis. This sequence of expression is consistent with tap being upstream of gcm in gliogenesis. In support of this hypothesis, the pattern of biparous expression is unchanged in gcm mutant embryos. Since TAP mRNA is expressed relatively early in neurogenesis, it is also possible that it is a downstream target of the proneural genes of the achaete-scute complex. To determine the effect of these genes on biparous expression, in situ hybridization experiments were performed with achaete-scute mutant backgrounds.