Synthesis of two mRNAs by utilization of alternate polyadenylation sites: expression of SV40-mouse immunoglobulin mu chain gene recombinants in Cos monkey cells.

The immunoglobulin mu gene encodes both membrane-bound (m) and secreted (s) mu chains. Two mRNAs, mum and mus, which differ only at their 3' ends and encode the two mu polypeptides, are produced differentially during maturation of B cells (bearing membrane-bound IgM) to plasma cells (secreting IgM). We have constructed a simian virus 40 (SV40)-mouse immunoglobulin mu chain chimeric recombinant carrying the SV40 early promoter and a part of the constant region of the mu gene, and also various deletion mutants of the recombinant. When the recombinant is introduced into Cos monkey cells, both mum and mus type mRNAs are synthesized. These two messages are polyadenylated at the mum and mus poly(A) addition sites, respectively. Correct splicing of the mu transcripts is also observed with reasonable efficiency. Analysis of the transcripts from the deletion mutants lacking either the mum or mus poly(A) addition site has indicated that the selection of the polyadenylation sites is determined simply by the proximity of the site to the promoter. This suggests that the machinery for 3' end formation of the messages, which probably determine the two alternate RNA processing pathways for mus and mum mRNAs, may scan in a promotor proximal to distal direction.