The Developmentally Regulated Shift from Membrane to Secreted μ mRNA Production Is Accompanied by an Increase in Cleavage-Polyadenylation Efficiency but No Measurable Change in Splicing Efficiency

To determine whether there are any developmental changes in the efficiencies of cleavage-polyadenylation or splicing reactions that could affect the usage of weak (suboptimal) processing signals and thus provide a basis for the regulated production of μ_m versus μ_s mRNA during B-lymphocyte maturation, we studied the expression of transfected μ genes in which the natural competition between cleavage-polyadenylation and splicing was replaced by alternative usage of tandem weak and strong poly(A) sites or by competition between suboptimal and optimal 5′ splice junctions. Our results indicate that there is a 50 to 100% increase in cleavage-polyadenylation efficiency but no measurable change in splicing efficiency as maturation proceeds from the B-cell to plasma cell stage.