The synapse between auditory Johnston's Organ neurons (JONs) and the giant fiber (GF) of Drosophila is structurally mixed, being composed of cholinergic chemical synapses and Neurobiotin-(NB) permeable gap junctions, which consist of the innexin Shaking-B (ShakB). Misexpression of one ShakB isoform, ShakB(N+16), in a subset of JONs that do not normally form gap junctions, results in their de novo dye coupling to the GF. This is similar to the effect of misexpression of the transcription factor Engrailed (En) in these same neurons, which also causes the formation of additional chemical synapses. In order to test the hypothesis that ShakB misexpression would similarly affect the distribution of chemical synapses, fluorescently-labeled presynaptic active zone protein (Brp) was expressed in JONs and the changes in its distribution were assayed with confocal microscopy. Both ShakB(N+16) and En increased the dye-coupling of