Avian DNA-binding receptor proteins

The avian steroid hormone receptor which has been most extensively studied is that for progesterone. It 

Glucocorticoid-response element Oestrogen-response element Thyroid-response element Dihydroxy vitamin Dj-response element COUP element (orphan-response element) 

The oestrogen receptor has also been purified from the domestic fowl oviduct, where it exists in three states (Raymoure et al., 1986). The three forms are interconvertible Rx, which has a high affinity for oestrogens Kd 0.06 nM), can be phosphorylated to Ry, a lower affinity form (K 0.8 nM), and Ry can be converted to a non-binding form (Rpb). 

25-dihydroxyvitamin Dj (l,25-(HO)2Dj) receptor has been purified from the small intestine of domestic fowl. The major component has a 

Phosphorylation of the l,25-(HO)2D3 receptor is induced by l,25-(HO)2D3 in embryonic chick duodenal organ culture. The phosphorylation occurs prior to initiation of Ca uptake and induction of synthesis of Ca -binding protein, which suggests a causal relationship (Brown DeLuca, 1990). Phosphorylation appears to be generally involved in the modulation of steroid receptor activity. 

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