THE ORPHAN RECEPTORS NGFI-B AND STEROIDOGENIC FACTOR-I ESTABLISH MONOMER BINDING AS A 3RD PARADIGM OF NUCLEAR RECEPTOR-DNA INTERACTION

We examined in detail the DNA interaction of the nuclear receptors NGF I-B and steroidogenic factor 1 (SF-1) by using a series of gain-of-fun ction domain swaps. NGFI-B bound with high affinity as a monomer to a nearly linear DNA molecule. The prototypic zinc modules interacted wit h a half-site of the estrogen receptor class, and a distinct protein m otif carboxy terminal to the zinc modules (the A box) interacted with two A/T base pairs 5' to the half-site. SF-1 bound in the same manner as NGFI-B, with an overlapping but distinct sequence requirement 5' to the half-site. The key features that distinguished the NGFI-B and SF- 1 interactions were an amino group in the minor groove of the SF-1 bin ding sequence and an asparagine in the SF-1 A box. These results defin e a common mechanism of NGFI-B and SF-1 DNA binding, which may underli e a competitive mechanism of gene regulation in steroidogenic tissues that express these proteins. This monomer-DNA interaction represents a third paradigm of DNA binding by nuclear receptors in addition to dir ect and inverted dimerization.