The monomer fluorescence of N-(1-pyrenyl)maleimide-labeled tropomyosin bound to F-actin (PTm-actin) increases when myosin subfragment 1 (S1) binds to actin and is half complete when only approximately 1 S1 is bound to 7 actin subunits [Ishii, Y., & Lehrer, S. S. (1985) Biochemistry 24, 6631-6638]. Similar studies of the binding of S1 and S1-ADP to fully reconstituted thin filaments [PTm-actin-troponin (Tn)] are now reported. The pyrene monomer fluorescence change was half complete when approximately 0.5 S1/7 actin subunits and approximately 1.5 S1/7 actin subunits were bound in the presence and absence of Ca2+, respectively. In the presence of Mg2+-ADP, when S1 binding is weakened, the S1 binding profiles and fluorescence changes were sigmoidal, with the cooperative transitions occurring at lower [S1] in the presence of Ca2+ as first shown by Greene and Eisenberg for S1 binding [Greene, L., & Eisenberg, E. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 2616-2620]. It was possible to fit both the binding and fluorescence data with the same parameters of a two-state (weak and strong S1 binding) cooperative binding model [Hill, T., Eisenberg, E., & Greene, L. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 3186-3190] for each Ca2+ situation if the fluorescence change is interpreted as the fraction of tropomyosin (Tm) units in the strong S1 binding state. These data indicate that the fluorescence change is a direct measure of the S1-induced change of state of Tm in the fully reconstituted thin filament.(ABSTRACT TRUNCATED AT 250 WORDS)