Nuclear Factors in B Lymphoma Enhance Splicing of Mouse Membrane-Bound μ mRNA in Xenopus Oocytes

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Abstract
Regulation of the synthesis of membrane-bound and secreted immunoglobulin .mu. heavy chains at the level of RNA processing is an important element for B cell development. The precursor .mu. RNA is either polyadenylated at the upstream poly(A) site (for the secreted form) or spliced (for the membrane-bound form) in a mutually exclusive manner. When the mouse .mu. gene linked to the SV40/HSV-TK hybrid promoter was microinjected into Xenopus oocytes, the .mu. messenger RNA (mRNA) was processed primarily to the secreted form. The processing pattern of .mu. mRNA was altered by coinjection of nuclei of mouse surface IgM-bearing B-lymphoma cells to include the synthesis of the membrane-bound form. An increase in the membrane-bound form was not observed when nuclei of IgM-secreting hybridoma cells or fibroblast cells were coinjected. Deletion of the upstream poly(A) site did not eliminate the effect of B-lymphoma nuclei suggesting that membrane-specific splicing is stimulated. Further, splicing of other .mu. gene introns was not affected by coinjection of .beta.-lymphoma nuclei. These results suggest that mature B cells contain one or more transacting nuclear factors that stimulate splicing specific for membrane-bound .mu. mRNA.