Functional interrogation of an odorant receptor locus reveals multiple axes of transcriptional regulation.

The odorant receptor (OR) genes constitute the largest mammalian gene family and are expressed in a monogenic and monoallelic fashion, through an unknown mechanism that likely exploits positive and negative regulation. We devised a genetic strategy in mice to examine OR selection by determining the transcriptional activity of an exogenous promoter homologously integrated into an OR locus. Using the tetracycline-dependent transactivator responsive promoter (tet(o)), we observed that the OR locus imposes spatial and temporal constraints on tet(o)-driven transcription. Conditional expression experiments reveal a developmental change in the permissiveness of the locus. Further, expression of an OR transgene that suppresses endogenous ORs similarly represses the OR-integrated tet(o). Neurons homozygous for the tet(o)-modified allele demonstrate predominantly monoallelic expression, despite their potential to express both copies. These data reveal multiple axes of regulation, and support a model of initiation of OR choice limited by nonpermissive chromatin and maintained by repression of nonselected alleles