In vitro biochemical experiments have led to predictions about the role of particular regions of TnI in regulation of contraction of muscle. To test whether these predictions hold in vivo , we have initiated a study of the
unc-27 gene in C. elegans , which is represented by three available alleles (
e155 ,
su142 sd, and
su195 sd). Mutants for the
unc-27 gene show either sluggish or no movement on bacterial lawns and display structural abnormalities in body wall muscle cells (Brenner. Genetics 77 :71-94, 1974; Zengel & Epstein. Cell Motil. 1 :73-97, 1980); transformation with a cosmid (ZK721) containing the C. elegans TnI-2 gene rescues the strain from its mutant phenotype (pers. com., R.J. Barstead). Analysis of swimming behavior of wild-type and
unc-27 homozygous adult hermaphrodites in nonviscous (S-basal buffer; rel. viscosity 1.0) and viscous (S-basal with 1% methyl cellulose; rel. viscosity 4.5) media revealed that
su142 sd is the most deleterious and that
su195 sd is the least. Sequencing showed that the mutant alleles code for prematurely terminated TnI-2: the recessive
e155 at residue Gln10, the semi-dominant
su142 sd at Gln122 (aligns with Lys87 of rabbit fast TnI), and the semi-dominant
su195 sd at Glu207 (aligns with Met170 of rabbit fast TnI). In situ localization of mRNA of TnI-2 and the three other C. elegans TnI genes explained the late onset of muscular defects associated with
unc-27 mutations, as well as the sparing of muscle groups other than the body wall. TnI-2 is expressed in larval and adult body-wall muscle, whereas TnI-1 is expressed in embryonic/larval body-wall muscle, as well as larval/adult reproductive muscles. TnI-4 is expressed in the pharynx. The rigid paralysis caused by
su142 sd confirms in vivo that the central region of TnI is needed for inhibition of tension development. The semi-dominance of the allele and the severity of paralysis it causes, relative to that caused by
e155 (a presumed null), suggest that TnI truncated just before the "inhibitory region" (aa 96-118 of rabbit fast TnI) is capable of competing against wild-type TnI-1 and TnI-2 for binding sites on the thin filament and of causing disinhibition/activation of contraction. Supported by Templeton Medical Research Foundation (TA) and NIH (EAB).