unc-42 mutants exhibit defects in locomotion (Unc) and response to anterior touch (Mec) as well as distinct axon pathfinding defects in neurons that form the left ventral nerve cord (VNC) bundle of C. elegans. Genetic mosaic experiments suggest that
unc-42 functions in motorneurons involved in locomotion and interneurons that mediate anterior touch. In
unc-42 mutants, the AVKR axon is often missing from the left VNC bundle, and the HSNL and PVQL axons leave the left VNC bundle and extend along axons in the right bundle. A similar HSNL phenotype is produced in laser-operated worms that lack the PVPR and PVQL interneurons which extend along the left VNC bundle. We previously reported YAC rescue of
unc-42 with Y42F5. To complete the cloning of
unc-42 we constructed a Lambda FixII library from Y42F5, isolated a 17 kb clone that rescued both
unc-42 and
smg-4 mutants, and identified a 7 kb minimal rescuing region for
unc-42. This region and corresponding cDNA clones encode a paired-type homeodomain protein. The
unc-42 homeodomain is similar to the homeodomains of the C. elegans genes
ceh-10 (70% identical),
unc-4 (60%),
vab-3 (56%) and
unc-30 (53%). The sequence of the mutant alleles confirmed this gene encodes
unc-42. The role of PVPR and PVQL in HSNL axon pathfinding and AVKR, HSNL and PVQL axon defects in
unc-42 mutants suggest that some of these neurons may express UNC-42 for proper pathfinding along the left VNC bundle. To determine if
unc-42 mutations transform the four neurons that contribute axons to the left VNC bundle, we examined differentiation markers for each neuron in
unc-42 mutants. The HSNs expressed the neurotransmitter serotonin, AVKR expressed the neuropeptide FMRFamide, the PVQs expressed the
sra-6::GFP transgene, and the PVPs expressed the UNC-30 protein (Y. Jin, personal communication) and MAb44 epitope (H. Bhatt & E. Hedgecock, personal communication), indicating that these neurons retain their cell identity. These results suggest that
unc-42 functions in cell differentiation, perhaps to regulate a subset of genes involved in axon pathfinding or fasciculation of one or more of the neurons that form the left VNC. To characterize UNC-42 protein expression, we are raising antibodies to UNC-42 fusion proteins and constructing worms that carry
unc-42::GFP and
unc-42::lacZ reporter genes.