REGULATION OF EXPRESSION WITHIN A GENE FAMILY - THE CASE OF THE RAT GAMMA-B-CRYSTALLIN AND GAMMA-D-CRYSTALLIN PROMOTERS

The six closely related and clustered rat gamma-crystallin genes, the gamma A- to gamma F-crystallin genes, are simultaneously activated in the embryonic lens but differentially shut down during postnatal devel opment with the gamma B-crystallin gene, the last one to be active. We show here that developmental silencing of the gamma D-crystallin prom oter correlates with delayed demethylation during lens fiber cell diff erentiation. Methylation silencing of the gamma D-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 proxim al activator. In later development, the gamma D-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 b y the lens. All gamma-crystallin promoters tested contain upstream sil encers, but at least the gamma B-crystallin silencer is distinct from the gamma D-crystallin silencer. The gamma-crystallin promoters were f ound to share a proximal activator (the gamma-box; around -50), which behaves as a MARE. The gamma B-box is recognized with much lower avidi ty than the gamma D-box. By swapping elements between the gamma B- and the gamma D-crystallin promoter, we show that activation by the gamma B-box requires a directly adjacent -46/-38 AP-1 consensus site. These experiments also uncovered another positive element in the gamma D-cr ystallin promoter, around -10. In the context of the gamma D-crystalli n promoter, this element is redundant; in the context of the gamma B-c rystallin promoter, it can replace the -46/-38 element.