Production of RNA for secreted immunoglobulin μ chains does not require transcriptional termination 5′ to the μ_M exons

Abstract

The mouse immunoglobulin μ gene encodes both secreted and surface-bound μ heavy chains produced by cells of the B lymphoid series. Transcripts of the μ gene are processed into μ mRNA species which differ at their 3′ termini, bearing either ‘μ_S’ or ‘μ_M’ segments, distinguishing secreted and cell-membrane-bound μ polypeptides^1–3. During maturation of surface IgM-bearing B cells to IgM-secreting plasma cells, both the total amount of μ mRNA and the ratio of μ_S to μ_M-terminated mRNA increase greatly^1,3,4. Two possible mechanisms for the developmental regulation of 3′ RNA processing cannot yet be distinguished. One mechanism would yield the μ_S terminus by specific cleavage of a common presursor transcript encompassing both μ_S and the μ_M exons (Fig. 1), the other by regulated termination of transcription upstream from the μ_M exons^2,5. While the first mechanism would produce, as a by-product, RNA fragments containing μ_M exons, the second would not. We report here the detection of such μ_M fragments in cells producing predominantly μ_S-terminated RNA species.