EXPRESSION OF THE UNC-4 HOMEOPROTEIN IN CAENORHABDITIS-ELEGANS MOTOR-NEURONS SPECIFIES PRESYNAPTIC INPUT

In the nematode, Caenorhabditis elegans, VA and Vp motor neurons arise from a common precursor cell but adopt different morphologies and syn apse with separate sets of interneurons in the ventral nerve cord. A m utation that inactivates the unc-4 homeodomain gene causes VA motor ne urons to assume the VB pattern of synaptic input while retaining norma l axonal polarity and output; the disconnection of VA motor neurons fr om their usual presynaptic partners blocks backward locomotion. We sho w that expression of a functional unc-4-beta-galactosidase chimeric pr otein in VA motor neurons restores wild-type movement to an unc-4 muta nt. We propose that unc-4 controls a differentiated characteristic of the VA motor neurons that distinguishes them from their VB sisters, th us dictating recognition by the appropriate interneurons. Our results show that synaptic choice can be controlled at the level of transcript ion in the post-synaptic neuron and identify a homeoprotein that defin es a subset of cell-specific traits required for this choice.