Hybridization kinetics of ribonucleic acid and deoxyribonucleic acid from antigen-stimulated mouse spleen cells.

In an attempt to explore the mechanism by which antigenic stimulation alters gene expression in lymphoid cells in vivo, three different hybridization techniques have been used to compare the complexity of the genome of lymphoid cells from normal and from immune BALB/c mice. RNA/DNA hybridization experiments at a DNA excess of 1000 demonstrated that normal RNA and immune RNA hybridized identically with DNA extracted from mouse spleen cells before and after immunization and with liver DNA. These findings indicate that DNA sequences complementary to immune RNA or to normal RNA are represented in a number of copies not significantly different in the genome of normal lymphoid cells and in that of immune lymphoid cells. Hybridizations of normal and of immune RNA with normal and immune pulse-labeled DNA, done at RNA excess, detected no differences between these two DNA. However, significant differences were observed in the percentage of DNA hybridized with normal and immune RNA; 3 to 4% of the DNA hybridized with normal RNA and 8 to 9% with immune RNA. This indicates that more DNA sequences are transcribed 48 hr after immunization than before immunization. The RNA exhaustion rates caused by normal and immune DNA were found to be identical, indicating that antigenic stimulation did not induce major changes in the number of DNA base sequences complementary to the RNA tested. However, when normal and immune pulse-labeled RNA were compared by exhaustion with DNA, the immune pulse-labeled RNA obtained 48 hr after immunization displayed a slower exhaustion rate than normal RNA or RNA extracted 72 hr after immunization. These results suggest the temporary synthesis, at a higher frequency, of certain RNA sepcies 48 hr after immunization, as compared to the RNA synthesis in normal, nonimmune cells, or that occurring 72 hr after immunization. Thus, the three experimental approaches used lead to the conclusion that antigenic stimulation does not induce major gene amplification; it does, however, change the transcription rate of certain RNA species.