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A suppressor mutation, D53, of theheld-up ² allele of the Drosophila melanogaster Troponin I (wupA) gene is described. D53, a missense mutation, S185F, of the tropomyosin-2,Tm2, gene fully suppresses all the phenotypic effects ofheld-up ², including the destructive hypercontraction of the indirect flight muscles (IFMs), a lack of jumping, the progressive myopathy of the walking muscles, and reductions in larval crawling and feeding behavior. The suppressor restores normal function of the IFMs, but flight ability decreases with age and correlates with an unusual, progressive structural collapse of the myofibrillar lattice starting at the center. The S185F substitution in Tm2 is close to a troponin T binding site on tropomyosin. Models to explain suppression by D53, derived from current knowledge of the vertebrate troponin-tropomyosin complex structure and functions, are discussed. The effects of S185F are compared …