Figure 8. Model for Combinatorial Coding of Odor Receptor Gene Choice.

Conserved gene-specific regulatory elements, the genes that contain them, and the maxillary palp ORNs in which the genes are expressed, are depicted. Green elements are positive regulatory elements; red, pink, and orange elements are negative. Solid green arrows connect genes to the ORNs in which they are expressed in wild-type. These arrows originate from regulatory elements in cases where the elements have been shown to act positively, but do not imply that such elements alone are capable of directing proper expression. By contrast, 42ax has not been tested functionally and its arrow does not originate from this element. Dashed green arrows indicate ectopic expression driven by the indicated elements in the context of a minimal promoter, expression that in wild-type is repressed through the agency of other indicated elements, as represented by the curved red, pink, and orange lines. The pink and orange elements and their positions in Or46a and Or59c, respectively, have not been defined. The prefix “pb” has been deleted from the ORN designations. In the ORNs are ovals representing predicted transcription factors, color-coded according to the elements upon which they are presumed to act. For simplicity, a single factor (dark green; left column, second row), expressed in pb1A, pb3A, and pb3B, is proposed to act on 59c1, and a single factor (light green; left column, top row) expressed in both pb2B and pb1A is proposed to act on 46a1; more complicated models in which these elements are acted upon by multiple factors are also possible. Promoters are not drawn to scale. AMC, antenno-maxillary complex, which contains the dorsal organs.