Worm Breeder's Gazette 15(2): 21 (February 1, 1998)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Department of Molecular Genetics, Albert Einstein College of Medicine, Bronx, NY 10461.
The enzyme tyrosine hydroxylase catalyzes the first step in the dopamine biosynthesis pathway. Expression of this protein is frequently used as an indicator of dopaminergic neuronal identity. We decided to use a similar strategy to follow the fate of dopaminergic neurons in C. elegans. To this end we isolated a putative tyrosine hydoxylase gene (TH) from C. elegans, identified in the genome sequence database on the basis of its sequence similarity to the Drosophila and rat TH genes. In the previous issue of WBG we described how gfp reporter genes constructed from the C. elegans TH gene are specifically expressed in all neurons previously defined as dopaminergic by Sulston et al. (1975). We also noted that the physical map location of C. elegans TH correlates closely to the genetic position of cat-2 (catecholamine abnormality), located on the left arm of Linkage Group II. cat-2 was originally identified by Sulston et al. (1975) in a genetic screen for mutants defective in dopamine biosynthesis or dopaminergic neuron development. The single cat-2 mutant isolated in this screen, e1112, fails to synthesize detectable levels of dopamine. To determine whether the C. elegans TH gene we identified corresponds to cat-2, we introduced the TH gene into the cat-2 mutant background and assayed for dopamine production using a formaldehyde induced fluorescence-based technique (Sawin, 1996). We observe that the presence of the TH gene in cat-2 animals restores dopamine to wildtype levels in the relevant neurons. To determine whether TH is mutated in cat-2(e1112) we sequenced independently derived PCR products from this strain. All contained a C to T transition that would introduce an ochre nonsense codon at amino acid position 161 in the predicted TH protein. This mutant protein would lack the catalytic domain essential for TH function. This defect suggests that e1112 is likely to correspond to a null mutation of TH which is consistent with the observation that dopamine production appears to be completely absent in cat-2 (e1112) animals (Sulston et al., 1975). Our finding that cat-2 encodes a putative tyrosine hydroxylase enzyme is consistent with the mutant phenotype. Unlike other cat mutants isolated in the screen by Sulston et al. (1975), cat-2 is specifically defective in the production of dopamine; serotonin levels are unaffected by mutations in this gene (Loer & Kenyon, 1993). We plan to use our TH/gfp reporter to study regulation of the cat-2 locus in various genetic backgrounds. As reported previously, we find that expression of TH in male rays depends upon a TGF-beta signal. References: Loer, C. M. & Kenyon, C. J. (1993) J. Neurosci. 13: 5407-5417. Sawin, E. R. (1996) Ph.D. thesis (MIT). Sulston, J. E., Dew, M., & Brenner, S. (1975) J. Comp. Neurol. 163: 215-226.