We have investigated whether different mRNAs of the thin filament are localized within a specific cytoplasmic compartment of muscle cells. It was postulated that co-localization of multiple thin filament mRNAs may allow the synthesis of the thin filament proteins within the same vicinity, which will enhance rapid assembly of the thin filament. This process would be more energy efficient compared to if the proteins were randomly diffused. We have investigated the cytoplasmic localization of [alpha]-actin, slow skeletal troponin C (sTnC) and [beta]-actin mRNAs in cultured mouse C2C12 myotubes, using in situ hybridization techniques. To address the mechanism of perinuclear localization of sTnC mRNA, we have characterized the 'cis'-acting localization signal of the sTnC mRNA. We have mapped the localization signal within a 40 nucleotide-long region of the 3'UTR. To further investigate how the localization signal works in vivo, we have studied its interaction with trans-acting factor(s) and found that a 40 kDa protein of muscle cells bind specifically to the localization signal of sTnC mRNA. (Abstract shortened by UMI.)