Transposon-induced deletions in unc-22 of C. elegans associated with almost normal gene activity

The unc-22 gene of Caenorhabolitis elegans encodes a protein which is a component of the myosin-containing A-band of the worm's striated body-wall muscle. Among 51 revertants of a transposon-induced mutant, we have identified four which retain a barely detectable mutant phenotype. Molecular analysis shows that three of these have in-frame deletions of 1.0, 1.3 and 2.0 kilobases, whereas the fourth partial revertant and two other apparently complete revertants have small insertions. All these rearrangements involve coding sequence and, in the case of the deletions, result in polypeptides that are shorter than the wild-type protein. The region of the gene containing these rearrangements contains 10 copies of a motif recognized in other regions of the gene (our unpublished data). We suggest that one explanation for the minimally mutant phenotype associated with the deletions is that the size and the repeated nature of the unc-22 protein structure make it relatively tolerant of substitutions or deletions involving one or a small number of repeated motifs. These results could explain why in some human genetic diseases, such as Duchenne's muscular dystrophy, deletions can be associated with only mild forms of the disease.