Regulated nuclear entry of the C. elegans Pax-6 transcription factor.

It is shown that the C. elegans Pax-6 locus encodes two protein isoforms. One contains a Paired DNA binding domain as well as a homeodomain; the other consists only of the carboxy-terminal portion of the locus encoding the homeodomain. These two isoforms are expressed in a variety of postembryonic cell lineages. In one set of lineages, nuclear localization of a homeodomain-only form (MAB-18 isoform) appears to be under temporal and spatial control. Nuclear localization of MAB-18 is correlated with the genetic requirement for mab-18 and with activation of a reporter gene driven by a mab-18 promoter. Reporter gene expression is dependent on mab-18 gene activity. It is hypothesized that a positive feedback loop is activated by regulated nuclear entry.