LENS-SPECIFIC GENE RECRUITMENT OF ZETA-CRYSTALLIN THROUGH PAX6, NRL-MAF, AND BRAIN SUPPRESSOR SITES

zeta-Crystallin is a taxon-specific crystallin, an enzyme which has un dergone direct gene recruitment as a structural component of the guine a pig lens through a Pax6-dependent mechanism, Tissue specificity aris es through a combination of effects involving three sites in the lens promoter. The Pax6 site (ZPE) itself shows specificity for an isoform of Pax6 preferentially expressed in lens cells, High-level expression of the promoter requires a second site, identical to an alpha CE2 site or half Maf response element (MARE), adjacent to the Pax6 site. A pro moter fragment containing Pax6 and MARE sites gives lens-preferred ind uction of a heterologous promoter, Complexes binding the MARE in lens nuclear extracts are antigenically related to IVrl, and cotransfection with Nrl elevates zeta-crystallin promoter activity in lens cells, A truncated zeta promoter containing Nrl-MARE and Pax6 sites has a high level of expression in lens cells in transgenic mice but is also activ e in the brain. Suppression of the promoter in the brain requires sequ ences between -498 and -385, and a site in this region forms specific complexes in brain extract. A three-level model for lens-specific Pax6 -dependent expression and gene recruitment is suggested: (i) binding o f a specific isoform of Pax6; (ii) augmentation of expression through binding of Nrl or a related factor; and (iii) suppression of promoter activity in the central nervous system by an upstream negative element in the brain but not in the lens.