Predicted to enable DNA-binding transcription factor activity, RNA polymerase II-specific and RNA polymerase II cis-regulatory region sequence-specific DNA binding activity. Involved in several processes, including defense response to Gram-negative bacterium; male mating behavior; and positive regulation of transcription by RNA polymerase II. Acts upstream of or within serotonin biosynthetic process. Located in nucleus. Expressed in several structures, including body wall musculature; excretory cell; intestine; neurons; and somatic nervous system. Is an ortholog of human RFX3 (regulatory factor X3).