Regulation of protein synthesis in mitogen-activated bovine lymphocytes. Analysis of actin-specific and total mRNA accumulation and utilization.

In response to the mitogenic lectin concanavalin A, the rate of protein synthesis in bovine small lymphocytes increased about 3-fold in 10 h and about 6-fold in 24 h. The rate of synthesis of the cytoskeletal protein actin, which comprises about 10% of total protein synthesis, increased in parallel with total protein synthesis. The cellular levels of actin mRNA were determined at various times after mitogen addition using a cDNA probe derived from a recombinant plasmid carrying a fragment coding for bovine actin. Actin mRNA sequences were found to increase significantly within 3 h and to increase 3.6 +/- 1.2-fold within 10 h after mitogen addition. Therefore, the increase in actin synthesis after mitogenic activation seems to be accounted for by accumulation of actin mRNA sequences. Total translatable mRNA and poly(A) sequences also accumulated in parallel with total protein synthesis, suggesting that regulation of actin synthesis was not unique. Two direct studies of mRNA utilization indicated that there is little, if any, translational regulation of actin synthesis. A detailed examination of actin mRNA-containing polysomes and total polysomes revealed no significant change in the average size of polysomes after cell activation. Therefore, given that the rates of polypeptide elongation and termination are constant after mitogen addition (Kay, J.E., Ahern, T., Lindsay V.J., and Sampson, J. (1975) Biochim. Biophys. Acta 378, 241-250), the rate of translational initiation per active mRNA must be unchanged. In addition, no significant amount of unutilized, nonpolysomal actin mRNA was detected in total cell extracts from either unstimulated or activated cells. Together these data suggest that elevated protein synthesis in activated lymphocytes is regulated by mRNA level.