Worm Breeder's Gazette 12(5): 88 (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.
We have been examining the organization of essential genes in the immediate vicinity of unc-22 (IV).The region of interest extends from the leftmost breakpoint of sDÄ8 to the rightmost breakpoint of sDÄ7. We are aware of the existence of a number of genes within this interval that we have been unable to mutate to yield recessive lethal phenotypes. In particular, in the 40 kb immediately to the left of un-22 there are at least five genes, including a Na+/ H+ antiporter-like gene, a DOPA decarboxylase-like gene, and a norepinephrine transporter-like gene. However, only one mutagenically defined locus, let-56 ,resides in this interval. Because we believe that many single copy C. elegans genes can mutate to yield recessive lethal phenotypes, and because we know we have not saturated the unc-22 region for mutations in essential genes, we have conducted a large screen for recessive lethals tightly linked to unc-22 ,and mapped these lethals with respect to sDÄ7 ant sDÄ8 (see figure blow). Over 9000 F1 chromosomeswere screened, and 160 lethal alleles were recovered on unc-22 ( s7 ) lev-1 ( x22 )marked chromosomes, and subsequently balanced over nT1 (IV;V).Fifteen lethal alleles previously isolated by D. Clark and M. Green in our lab were also analyzed. Of these 175 lethal alleles, nineteen failed to complement sDÄ7 and/or sDÄ8.
Seven of these nineteen alleles failed to complement sDÄ7, but complemented sDÄ8. Therefore, these alleles fall into zones which currently contain only two lethal alleles. One of these lethal alleles defines let-52 ,and the other defines let-93 . Iet-52 and let -93 are separated by the rightmost breakpoints of sDÄ9 and sDÄ65, so we presume they are distinct genes. With our seven newly generated alleles, we have more than tripled the number of lethal alleles between the rightmost breakpoints of sDÄ7 and sDÄ8. Because both let-52 and let-93 appear to be small targets for EMS mutagenesis, it is possible that lethal alleles defining a new gene or genes have been recovered.
Ten of the nineteen alleles fail to complement both sDÄ7 and sDÄ8. Prior to this work, the region of overlap between sDÄ7 and sDÄ8 contained 11 alleles defining three genes. Therefore, we have almost doubled the number of alleles mapping to this region. Two of the new alleles fail to complement let-56 ,making let-56 the largest essential gene EMS target in this region, with a total of 10 alleles. One new allele fails to complement let-653 ,which was previously defined by a single allele. Another new allele, when homozygous, exhibits the distinctive let-653 "hole-in-head" phenotype (S. Jones, personal communication) and is presumed to represent another allele of let-653 .
One new allele mapped to a zone containing only let-92 ,yet complemented let-92 .This new allele was assumed to define a new gene and was designated let-660 .This assumption was later shown to be valid when a new deficiency, sDÄ66, was found to delete let-660 but not let-92 ,placing the two genes in different zones. The other alleles mapping to this region are currently under snalysis.
Two of the nineteen alleles fail to complement sDÄ8 but complement sDÄ7. That so few of the nineteen alleles map here is somewhat surprising, as let-65 maps to this region, and it is a large target for EMS mutagenesis (nine alleles). Hermaphrodites homozygous for one of the new alleles reach adulthood, and produce a few eggs which are not laid and do not hatch. This phenotype is consistent with that observed for let-65 .Animals homozygous for the other of these alleles are arrested in development at approximately the L2 stage. This allele may be another mutation in let-60 .Additional complementation tests are underway to examine these possibilities.