After this large burst of expression, TAP mRNA is restricted to progressively fewer cells. ELAV is an RNA binding protein expressed in all postmitotic neurons. These experiments reveal that TAP mRNA is largely excluded from ELAV-positive cells. This is true even at stage 15 when the vast majority of cells in the CNS have reached the postmitotic state. Further confirmation of the absence of TAP staining from neurons was obtained using anti-HRP antiserum, which recognizes all neurons.
Cells express repo after they have committed to the glial cell fate.
Later in neurogenesis when the two genes do overlap, most TAP -positive cells are REPO negative. Therefore, the possibility that in some cases expression of the two genes might overlap for a very short time in the same cell cannot be excluded. Since biparous-positive cells express neither elav nor repo , it is possible that they are not yet postmitotic. To follow the fate of the TAP -expressing cells, a fusion gene was constructed in which 5.5 kb of the tap promoter drives expression of lacZ . It was reasoned that perdurance of beta-galactosidase would permit observation of cells that had turned on biparous even after disappearance of the transcript.