CDNA SEQUENCES AND ORGANIZATION OF IGM HEAVY-CHAIN GENES IN 2 HOLOSTEAN FISH

Immunoglobulin M heavy chain (mu) sequences of two holostean fish, the bowfin, Amia calva, and the longnose gar, Lepisosteus osseus, were am plified from spleen mRNA by RACE-PCR, cloned, and sequenced. Each mu c hain showed the conserved four constant domain structure typical of a secreted mu chain. Southern blot analyses with specific heavy chain va riable (V-H) and constant (C-H) region probes suggest that both fish p ossess an IgH locus that resembles that of the teleosts, amphibians, a nd mammals in its organization. The overall sequence similarity of gar and bowfin mu chains was 60% and 48% at the nucleotide and amino acid levels, respectively, while similarity to the mu chains of teleosts a nd elasmobranchs was lower. The bowfin mu chain possesses a distinctiv e proline-rich sequence at the C mu 1/C mu 2 boundary; a shorter proli ne-rich sequence is present at this position in the gar mu chain. Both gar and bowfin show, in their C mu 4 sequences, motifs that could ser ve as cryptic splice donor sites for the production of mRNAA encoding the membrane-bound form of the mu chains, and the bowfin also shows a potential cryptic splice donor site in the C mu 3 exon.