(A) Electrophoretic mobility shift assay of MEF2 interacting with three different probes: Actin57B control (lanes 1–3), sing315-1 (lanes 4–7), and sing 315-2 (lanes 8–11). Wild type competitor was used in lanes 3, 6, and 10, and mutant competitor was used in lanes 7 and 11. MEF2 bound to the two sites in the sing enhancer and this interaction was sequence-specific, since wild-type sequences competed the interaction, whereas mutant sequences did not compete the interaction. The smear below the shifted band probably represents a minor modified or breakdown isoform of MEF2 interacting with the DNA. (B–B“) Horizontal section of 24h APF sing-lacZ animals stained to visualize MEF2, DAPI, and βGal in adult myoblasts. Note the accumulation of the βGal reporter in myoblasts. (C–C“) Horizontal section of 24h APF transgenic animals carrying sing-lacZ with both MEF2 binding sites mutated. Sections were stained as in B. Note the absence of βGal staining. (D–D“) Horizontal section of 24h APF 1151>dcr + Mef2-RNAi animals carrying sing315-lacZ. βGal staining was diminished in the absence of MEF2. Scale bar, 20µm.