Regulation of expression within a gene family. The case of the rat gammaB- and gammaD-crystallin promoters.

The six closely related and clustered rat gamma-crystallin genes, the gammaA- to gammaF-crystallin genes, are simultaneously activated in the embryonic lens but differentially shut down during postnatal development with the gammaB-crystallin gene, the last one to be active. We show here that developmental silencing of the gammaD-crystallin promoter correlates with delayed demethylation during lens fiber cell differentiation. Methylation silencing of the gammaD-crystallin promoter is a general effect and does not require the methylation of a specific CpG, nor does methylation interfere with factor binding to the proximal activator. In later development, the gammaD-crystallin promoter is also shut down earlier by a repressor that footprints to the -91/-78 region. A factor with identical properties is present in brain. Hence, a ubiquitous factor has been recruited as a developmental regulator by the lens. All gamma-crystallin promoters tested contain upstream silencers, but at least the gammaB-crystallin silencer is distinct from the gammaD-crystallin silencer. The gamma-crystallin promoters were found to share a proximal activator (the gamma-box; around -50), which behaves as a MARE. The gammaB-box is recognized with much lower avidity than the gammaD-box. By swapping elements between the gammaB- and the gammaD-crystallin promoter, we show that activation by the gammaB-box requires a directly adjacent -46/-38 AP-1 consensus site. These experiments also uncovered another positive element in the gammaD-crystallin promoter, around -10. In the context of the gammaD-crystallin promoter, this element is redundant; in the context of the gammaB-crystallin promoter, it can replace the -46/-38 element.