Tropomyosin (TM) is an important actin binding protein involved in regulation of muscle contraction. Unacetylated striated tropomyosin failed to bind to actin whereas unacetylated smooth tropomyosin bound well to actin. It has been demonstrated that high actin affinity of unacetylated ${alpha}-tropomyosin$ was ascribed to the carboxyl terminal amino acid residues. In order to define the role of the carboxyl terminal residues of tropomyosin molecule on actin binding, two mutant tropomyosins were constructed. TM11 is identical to the striated tropomyosin except that the carboxyl terminal last three amino acids was replaced with $^{282}NNM^{284}$ whereas in TM14 $^{276}HA^{277}$ was substituted with smooth specific $^{276}QT^{277}$. TM11 and TM14 were overproduced in Escherichia coli and analyzed for actin affinity. The apparent binding constants (Kapp) of unacetylated tropomyosins were $2.2{ imes}10^6M^{-1}$ for sm9, $1.03{ imes}10^6M^{-1}$ for TM14, $0.19{ imes}10^6M^{-1}$ for TM11, $>0.1{ imes}10^6M^{-1}$ for striated, respectively. This result indicated that higher actin affinity of the unacetylated smooth tropomyosin was primarily attributed to the presence of QT residues in the smooth sequence. In case of the Ala-Ser (AS) dipeptide extension of the amino terminus of tropomyosin, Kapp were $21.1{ imes}10^6M^{-1}$ for AS-sm9, $8.0{ imes}10^6M^{-1}$ for AS-11, $4.7{ imes}10^6M^{-1}$ for AS-14, $3.8{ imes}10^6M^{-1}$ for AS-striated. AS-TM11 showed considerably higher actin affinity than AS-TM14, implying that interaction of Ala-Ser of the amino terminus with the carboxyl terminal residues. Since Kapp of AS-TM11 was significantly lower than that of AS-sm9, the presence of QT might be required for restoration of high actin affinity of the smooth ${alpha}-tropomyosin$. These results suggested that the carboxyl terminal amino acid residues Glutamine275-Threonine276 are important for actin affinity of the recombinant smooth ${alpha}-tropomyosin$, particularly of unacetylated smooth ${alpha}-tropomyosin$.