Sequence variability at the N-terminal leader peptides of rat immunoglobulin light chain precursors

The mRNA molecules coding for immunoglobulin κ chains of different subgroups were isolated from three rat myeloma tumors (IR102, IR487 and IR52). The mRNAs directed the cell-free synthesis of precursor molecules in which leader peptides (24-20 residues long) preceded the N-termini of the mature L-chains. All precursors had N-terminal initiator methionine residue. The complete primary structure of the IR102 leader, and the partial sequences of the IR487 and IR52 leaders were determined. The leader peptides differed extensively in sequence (> 37%), similarly to mouse Ig leader peptides of different subgroups. However, the rat leaders had the invariant residues Gly^?4, Leu^?9, Leu^?10 and Met^?20 (observed in mouse), that presumably are functionally significant and are conserved in evolution (Schechter et al., Fed. Proc. 1979, $\text{38}$, 1839). We predict that in rat genome, as in mouse genome, the DNA coding for the leader and V-region are not contiguous, and that the invariant Gly^?4 codon marks the 3′ end of the leader coding segment.