Evolutionary conservation of splice sites in sterile C mu transcripts and of immunoglobulin heavy chain (IgH) enhancer region sequences.

The immunoglobulin JHC mu intron was cloned from genomic DNA of a VHa3 rabbit and a 1257 bp sequence which contains conserved enhancer and splice sites was determined. From positions 315 to 1257, there is approximately 72 and 67% similarity to available sequences of man and mouse, respectively (counting gaps as single changes at single positions). In earlier studies of rabbit cDNAs encoding immunoglobulin heavy chains, we found a C mu-encoding cDNA clone (pB3) derived from splenic mRNA of a Trypanosome-hyperimmunized rabbit (VHa1) which lacked VH, DH or JH sequences and had an unknown sequence 5' of that encoding C mu. Comparison of this cDNA sequence with the present cloned genomic DNA sequence has now revealed that the start of cDNA pB3 corresponds to a position 80 base pairs 3' of the conserved octamer motif of the rabbit heavy chain enhancer. This mRNA was spliced to the acceptor site of C mu using a donor site which was 635 bp 3' of the enhancer octanucleotide. Our sequence of pB3 indicates that in rabbit as in mouse, a "nontron" (33 stop codons in three reading frames) can be formed utilizing a conserved splice site to produce a spliced transcript. The presence of evolutionarily conserved splice donor sites in the intron sequences of rabbit, mouse and man suggests a functional role during B cell ontogeny.