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

Abstract

The immunoglobulin J HCμ intron was cloned from genomic DNA of a V Ha3 rabbit and a 1257 bp sequence which contains conserved enhancer and splice sites was determined. From positions 315 to 1257, there is ~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 Cμ-encoding cDNA clone (pB3) derived from splenic mRNA of a Trypanosome-hyperimmunized rabbit (V Ha1) which lacked V H, D H or J H sequences and had an unknown sequence 5' of that encoding Cμ. 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 Cμ 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.