Presence of a polyadenylated RNA fragment encoding the membrane domain for immunoglobulin alpha chain indicates that mRNAs for both secreted and membrane-bound alpha chains can be produced from the same RNA transcript.

RNA blotting was employed to examine polyadenylated immunoglobulin alpha chain RNAs in a B lymphoma synthesizing membrane-bound and secretory IgA and in a hybridoma which synthesizes predominantly secretory IgA. Both cell lines were derived from the I.29 lymphoma and expressed the identical heavy chain variable region gene. In addition to the predicted mRNA precursors, four novel species of polyadenylated alpha RNAs were detected. The presence of a RNA species which was too large to have the same 3' end as the largest mRNA for membrane-bound alpha chain (alpha m) implied that transcription continued past the alpha m poly(A) site, and that such transcripts could be polyadenylated. Alternatively, transcription of this alpha RNA was initiated 5' to the normal cap site. Two species of RNA were detected which encoded the alpha m domain and the intervening sequence between the alpha constant (C alpha) and alpha m domain but not the C alpha domain. These RNA molecules were of sizes appropriate for their derivation by endonucleolytic cleavage of a precursor for alpha m mRNA at the poly(A) site of the mRNA for secreted alpha chains (alpha s). The presence of these three alpha RNA species suggested that alternative and successive cleavage/polyadenylation events could occur on a single transcript to produce either alpha m or alpha s mRNAs. An additional novel species of RNA was detected which indicated that the order of removal of the large IVSs did not always proceed in the 5' to 3' direction.