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Stimulation of Murine B Lymphocytes Induces a DNA Exonuclease Whose Activity on Switch-µ DNA Is Specifically Inhibited by Other Germ-Line Switch Region RNAs¹

Jürgen R. Müller^*, and Kenneth B. Marcu^2,

^* Laboratory of Genetics, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892; and Department of Biochemistry and Cell Biology, State University of New York, Stony Brook, NY 11794

The Ig heavy chain class switch in B lymphocytes involves a unique genetic recombination that fuses specific regions within the Ig locus and deletes intervening sequences. Here we describe a novel exonuclease activity in nuclear lysates of B cells in an in vitro assay. This activity was induced in B lymphocytes after treatment with either LPSs or CD40 ligand/anti--dextran, both of which induce switch recombination, and considerably less activity was detected in untreated or anti--dextran-treated B cells, Con A-stimulated spleen cells, liver cells, or a number of cell lines. The exonuclease activity was dependent on divalent cations, and both 3' and 5' labels were efficiently removed from DNA substrates. The presence of RNase A, but not RNase H, inhibited exonucleolytic digestion, suggesting that a ribonucleoprotein is responsible for the exonucleolysis. The DNA digestion appears to be nonspecific, since DNA substrates with either switch-µ or unrelated sequence were hydrolyzed with comparable efficiency. Germ-line switch region transcripts (Ig1, Ig3, and Ig) strongly inhibited the exonucleolysis of switch-µ DNA but not that of unrelated control DNA, while switch antisense RNA or tRNA were much less effective inhibitors.

This article has been cited by other articles:

				J. Ballantyne, D. L. Henry, J. R. Muller, F. Briere, C. M. Snapper, M. Kehry, and K. B. Marcu Efficient Recombination of a Switch Substrate Retrovector in CD40-Activated B Lymphocytes: Implications for the Control of CH Gene Switch Recombination J. Immunol., August 1, 1998; 161(3): 1336 - 1347. [Abstract] [Full Text] [PDF]