C. M. Loer* and S. A. Anderton. Dept. of Biology, Lafayette College, Easton, PA 18042 and *Dept. of Biology, University of San Diego, CA 92110 (current address).
We are interested in learning how neurons in C. elegans decide to use the neurotransmitter serotonin. We are using two approaches to address this question: 1) isolation and characterization of serotonin-deficient mutants, which may identify regulatory genes or serotonin synthetic enzyme genes, and 2) identification and characterization of synthetic enzyme and other terminal differentiation genes used by serotonergic neurons, which are the genes controlled by the regulators of neurotransmitter type.

Serotonin synthesis is catalyzed in two steps from tryptophan: Trp is first hydroxylated by an aromatic amino acid hydroxylase (AAAH), then decarboxylated by an AAA decarboxylase (AAADC) to yield serotonin. We have identified genes encoding both enzymes in C. elegans from ORFs first predicted by the C. elegans Genome Sequencing Consortium. A sequence with high homology to mammalian Trp hydroxylase is found on cosmid ZK1290. We made lacZ reporter fusion constructs with the predicted ORF; transgenic worms containing these constructs expressed the gene in a subset of identified serotonergic neurons, including NSMs, HSNs, some CPs and others. A tandemly repeated pair of ORFs encoding AAADC-homologous proteins is found on cosmid C05D2, which mapped near the bas-1 gene, mutants of which are serotonin-deficient (Loer & Kenyon, J. Neurosci. 13: 5407, 1993). Injection of C05D2 cosmid rescued the serotonin-deficiency of bas-1 mutants. We are interested in determining whether one or both of these ORFs, only 369 bp apart, is needed for serotonin synthesis. We are also examining 5' upstream regions of both the ZK1290 and C05D2-encoded genes for common regulatory elements that may be used by factors that specify a serotonergic phenotype. This work is supported by NSF (RUI) Grant IBN94-10974 to CML.