Structural analysis of the murine IgG3 constant region gene.

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Abstract
We have determined the complete sequence of the gamma 3 heavy chain constant (C gamma 3) region gene of the BALB/c mouse including the 5′‐flanking region up to the switch site and the 3′‐flanking region past the end of the membrane exons. The C gamma 3 coding region, typical of other IgGs, is divided into six exons corresponding to the protein domains (C gamma (3)1, hinge, C gamma (3)2, and C gamma (3)3) and to the membrane carboxyl terminus (M1 and M2). The predicted amino acid sequence of the gamma 3 chain has three potential N‐linked carbohydrate addition sites (including one in the membrane spacer segment), as compared with a single occurrence in the other mouse IgGs. Between the switch recombination region and the body of the C gamma 3 gene, there is a remarkable homology with a sequence between C mu and C delta which provides a rationale for an alternative, T cell‐independent class‐switch mechanism. We have used a computer to analyze the secondary structure of the gamma 3 mRNA precursor for the membrane form. We predict that this RNA precursor (approximately 12 000 bp) folds into four leaf‐like domains which correspond to the variable region, the large IVS, the body of the constant region, and the membrane exons. This organization may have a role to play in the function of the mRNA precursor.