Figure 2.

A: Comparison of palindromic P3 motif variants (top) in phototransduction genes that are broadly expressed in all Drosophila photoreceptors with RCSI motif variants (Rhodopsin Core Sequence I, bottom) in rhodopsin genes (Rh1-Rh6) whose expression is restricted to different subsets of photoreceptors.

Top, P3 motif variants in broadly expressed phototransduction genes contain Q50 motifs (TAATTA/G, blue) and have a palindromic repeat of the homeodomain core motif (TAAT…ATTA).

Bottom, each rhodopsin gene ‘prefers’ a specific imperfectly palindromic RCSI motif variant. Note that Rh3 and Rh6 contain K50 motifs (TAATCC, green). Only the 5’ TAAT core motif of rhodopsins matches the palindromic repeat in broadly expressed phototransduction genes (dashed box), while the inverted 3’ repeat is imperfect in Rh1-Rh5 (indicated by purple letters: ATTT, GTTA, or ATTC instead of ATTA).

B: Left, a homodimer of the Q50 homeodomain transcription factor Pph13 binds to a palindromic P3 motif (that contains a Q50 motif, TAATTG) in broadly expressed phototransduction genes. Right, the activator Pph13 (blue) is expressed in all photoreceptors (R) of the unit eye and drives broad expression of phototransduction genes.

C: Left, the central base pairs of the RCSI motif differ in some rhodopsin genes, for instance in Rh6. This generates a K50 motif (TAATCC) that recruits the K50 homeodomain transcription factor Otd. Right, like Pph13 (blue), the activator Otd (green) is expressed in all photoreceptors of the unit eye and drives broad expression of its target genes.

D: Left, the same K50 motif (TAATCC) recruits the K50 homeodomain transcription factor Dve, which is a repressor. Right, the repressor Dve (red) is expressed in ‘outer’ photoreceptors (R1-R6) where it prevents activation of Rh6 by Pph13 and Otd. However, the ‘inner’ photoreceptor R8 does not express Dve and therefore Pph13 and Otd can activate Rh6 in this subset of photoreceptor neurons.