Distribution of lymphocyte messenger RNA during stimulation by phytohaemagglutinin.

Additional of phytohaemagglutinin to cultured lymphocytes results in a progressive increase in the rate of protein synthesis of up to 7-10 times the initial rate after 20 h. Between 2 h and 12 h after the addition of mitogen, the increase in the rate of protein synthesis can be accounted for by the transfer of mRNA from messenger ribonucleoprotein complexes to polysomes. Between 12 h and 20 h the increase also reflects the accumulation of mRNA and ribosomes. However, the proportion of mRNA associated with ribosomes in unstimulated lymphocytes is unexpectedly high and paradoxically decreases during the first 2 h after mitogen addition, although the rate of protein synthesis increases. A mechanism involving mRNA selection is suggested.