Abstract - A preparation from rabbit muscle was found to incorporate leucine and serine into endogenous troponin. Leucine was attached to troponin by a covalent bond. The incorporation was time dependent, temperature dependent, and highly specific suggesting an enzyme is involved in bond formation. All inhibitors of protein synthesis, except for puromycin and chloroamphenicol at high concentration, failed to Inhibit incorporation. Incorporation was also not dependent on ATP or GTP which inhibited the system at all concentrations. Electrophoresis of troponin with bound leucine revealed the incorporation of leucine into the inhibitory subunit of troponin. Leucine bound to troponin could be released by boiling in 1N HC1 or 1 N NaOH. Acid hydrolysis revealed that leucine was modified before Incorporation. Leucine could not be removed from troponin by pronase, periodate, or carboxypeptidase A. Bound leucine could be dansylated. Leucine was interpreted to bind to troponin by an ester bond through the carboxyl group of leucine. Alakline phosphatase strongly inhibited leucine incorporation into troponin. Phosphatase activity was present in the troponin preparations. The phosphate data suggests that leucine incorporation may be regulated by cyclic AMP. A fraction isolated from troponin on DEAE cellulose inhibited the incorporation of leucine. The preparation failed to release leucine bound to troponin, proteolysis was not involved in Incorporation. The Incorporation of serine into troponin altered the degree of calcium induced conformational change of the molecule. Troponin-serlne acquired the same level of conformational change as troponin with one-fourth the concentration of calcium. Three mechanisms were proposed, to describe leucine incorporation. All three were activated by cAMP. In the first mechanism a cAMP dependent kinase activates the incorporating enzyme by the addition of a phosphate group. The active enzyme then modifies the amino acid and Incorporates the amino acid into the inhibitory subunit of troponin using an ester bond. The second mechanism involves the Incorporation of an amino acid, via phosphoserine hydrolysis, into the inhibitory subunit. The last mechanism proposes that the phosphate group on the inhibitory subunit of troponin acts as a marker and/or signal for the enzyme catalysing amino acid incorporation. All three mechanisms result in the same product between the amino acid and troponin. [...] Abstract. The log of poly(A) size and the log of mRNA half-life for different organisms was found to be related in a linear manner. Predictions of specific mRNA half-lives and poly(A) size were made to help show a direct relationship. This relationship was also valid for histone poly(A) minus mRNA provided DNA synthesis is interrupted. A mechanism of mRNA degradation was proposed which is regulated by poly(A) size and allows a stochastic decay of the messages. [...]