Worm Breeder's Gazette 12(5): 71 (February 1, 1993)

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.

Dominant Mutations in unc-37 Suppress the unc-4 Neuronal Wiring Defect (or bkn-1 Bites the Dust)

David M. Miller, Charles J. Niemeyer, Pritha Chitkara

Department of Cell Biology, Duke Univ. Medical Ctr, Durham, NC 27710

We are interested in the question of how neurons choose synaptic partners. In unc-4 mutants, VA motor neurons receive synaptic input normally reserved for their sister cells the VB motor neurons. As a result of the lost input to the VA's, unc-4 mutants are unable to crawl backwards (1). unc-4 encodes a homeodomain protein (2) that is expressed in VA but not VB motor neurons (WBG 12, #3, p80 )which suggests that unc-4 may regulate the expression of some feature of the VA's that allows presynaptic interneurons to distinguish them from their VB lineage siblings. We have previously described a selection strategy for isolating suppressors of the temperature sensitive allele unc-4 ( e2322ts )(WBG 12, #3, pp 107-108) as a means of identifying the presumptive unc-4 target gene. [ unc-4 ( e2322ts )contains a Leu to Phe substitution in the unc-4 homeodomain.] Four dominant, extragenic suppressors and four dominant, intragenic revertants were isolated. All of these suppressors/revertants appear essentially wild type in an unc-4 (ts)background which suggests that the normal pattern of input to VA motor neurons has been restored. The extragenic suppressors are within 0.07 map units to the left of unc-87 suggesting that they are allelic. This interval includes the unc-37 locus. Sydney Brenner isolated unc-37 ( e262 )and described it as a "coiler." We have noticed that unc-37 ( e262 )is also a phenocopy of unc-4 (0)mutants; neither can crawl backwards and both coil dorsally (vulva on outside of loop) when touched on the head. This suggested that the unc-4 suppressors are likely to be unc-37 (gf)mutations. To test this idea, we screened for revertants of the suppressor unc-37 ( wd17 ).EMS-mutagenized dpy-5 ( e61 ) unc-37 ( wd17 )I; unc-4 ( e2322ts )IIhermaphrodites were mated at 25 C with unc-4 ( e2322ts )males [grown at 16 C]. From ~10,000 F1 'swe obtained two revertants, unc-37 ( wd17 wd19 )and unc37 ( wd17 wd20 ).Both fail to complement unc-37 ( e262 ).We propose therefore, that the unc-4 suppressor mutations are in fact, unc-37 (gf)alleles. unc37 ( wd17 wd19 )and two other recently isolated unc-37 alleles (3) are recessive lethals which suggests that unc-37 (0)is lethal. Perhaps unc-37 corresponds to an unc-4 target gene encoding a cell surface receptor expressed in VA motor neurons and recognized by specific presynaptic interneurons. Alternatively, unc-37 could encode a cofactor that interacts with the unc-4 homeodomain. In either case, however, unc-37 is also likely to provide essential functions in other cell types as well. We are now microinjecting cosmids to rescue unc-37 ( e262 ).

Literature Cited:

1) White et al. (1992) Nature 355, 838-841.

2) Miller et al. (1992) Nature 355, 841-845.

3) McKim and Rose (1992) Mol. Gen. Genet., 233, 241-251.