The Transfer RNA Methylases of Human Lymphocytes. I. Induction by PHA in Normal Lymphocytes

Assays were performed that measured both the rate and extent (or total capacity) of transfer RNA methylases in extracts of lymphocytes cultured in the presence and absence of phytohemagglutinin (PHA). The tRNA methylases of human peripheral blood lymphocytes undergoing blastogenesis in culture with PHA had a five- to sixfold increase in rate and a three- to sevenfold increase in extent of methylation of heterologous tRNA. These data suggest that PHA transformed lymphocytes not only contain elevated levels of tRNA methylases, but that the increase includes qualitatively different enzymes from those found in normal peripheral blood lymphocytes. Experiments in which lymphocytes were incubated for various times with PHA revealed that tRNA methylase induction occurred late in or after DNA synthesis and after morphologic transformation, but prior to mitosis. Rate and extent of tRNA methylation increased simultaneously. PHA induction of tRNA methylase activity was dependent on the synthesis of new RNA in lymphocytes cultured from 40 to 45 hours. The increase was not due to different levels of inhibitors or activators or preferential degradation of reaction components. The data suggest that quantitative and qualitative changes occur in the tRNA methylases of the normal human peripheral blood lymphocyte stimulated by PHA to undergo transition to an undifferentiated cell "in cycle." The possible significance of these findings to control of protein synthesis in PHA transformed lymphocytes is discussed.