Suppressor of Hairless (Su (H)) is required at two stages of adult sensory organ development in Drosophila. Complete loss of Su (H) function results in a ‘neurogenic” phenotype in lmaginai discs, in which too many cells adopt the sensory organ precursor ceil fate. Su (H) Is also involved in controlling the fates of sensiiium accessory cells and is specifically expressed in two of these cells. Su (H) is the Drosophila homoiog of the mouse J, RBP gene, whose product binds specifically to the recombination signal sequence of immunoglobulin J, segments. The Su (H) and J, RBP proteins are 82% identical over most of their length, and share with bacteriophage integrases and yeast recombinases a motif that includes residues directly involved in catalyzing recombination. introduction

The adult peripheral nervous system (PNS) of Drosophila is composed of several types of sensory organs, or sensilla, that are distributed on the body surface in a largely invariant spatial pattern. An individual sensillum is composed of several ceils that in most cases are the progeny of a single sensory organ precursor (SOP) cell (Hartenstein and Posakony, 1989). The SOPS appear during the late larval and early pupal stages within the imaginal discs and histoblast nests, undifferentiated epithelial sheets that will give rise to the cuticular structures of the adult fly. The determination of the SOP cells appears to take place in a stepwise fashion (Ghysen and Dambly-ChaudiBre, 1989). First, the spatially restricted expression of the transcriptional regulatory proteins encoded by the echaete and scute genes defines clusters of ceils that are competent to become SOPS (Cubas et al., 1991; Skeath and Carroll, 1991). Subsequently, one cell, the SOP, is singled out of each cluster, and the remaining cells are inhibited from expressing the SOP fate by local cell-cell interactions. This inhibitory signaling requires the activity of at least five loci of the neurogenic group (Notch [w, Delta [DJ, Enhancer of split [E (spl)], mastermind [rnam], and neuralized [neu]). Loss-of-function mutations in the neurogenie genes cause the emergence of supernumerary SOPS within the competent clusters, and these give rise to groups of sensory organs around the normal positions of single sensilla (Dietrich and Campos-Ortega, 1984; Hartenstein and Posakony, 1990a; Simpson, 1990). Following its determination, the SOP follows a stereotyped pattern of division to generate the component cells