ALTERNATE PRE-MESSENGER-RNA PROCESSING PATHWAYS IN THE PRODUCTION OF MEMBRANE IGM HEAVY-CHAINS IN HOLOSTEAN FISH

A single gene encodes two forms of the IgM heavy chain (mu) in vertebr ates: one (mu s) present in serum as secreted IgM and the other (mu m) as the antigen receptor form of IgM present on the B-lymphocyte membr ane. The mRNAs encoding mu s and mu m are derived from a single primar y transcript by alternate pathways of RNA processing. In all vertebrat es so far examined, with the exception of teleosts, mu m mRNA is produ ced by splicing the transmembrane (TM) encoding exon 1 into a cryptic donor site near the 3' end of the C mu 4 exon. In contrast, teleost sp ecies splice the TM exon 1 into the regular splice donor site at the 3 ' boundary of the C mu 3 exon, We have examined mu m mRNAs in two spec ies of primitive bony fish, the holostean bowfin and the longnose gar. These fish utilize both the C mu 3 to TM1 (teleost) pathway and the t ypical cryptic C mu 4 to TM1 pathway. In addition the bowfin possesses a cryptic splice donor site near the middle of C mu 3. This is used i n the production of a third species of mu m-encoding mRNA, but does no t participate in the production of an alternate form of the mu s mRNA, The structure and patterns of expression of their mu genes suggest th at the gar and bowfin may be more closely related than implied by the current view of fish evolution.