Diversity of expressed V and J regions of immunoglobulin light chains in Xenopus laevis

Abstract

In Xenopus laevis, two immunoglobulin light chain isotypes, designated L1 or rho and L2 or sigma, have been identified. The genomic organization of the L1 locus has been described previously: a constant (C) gene segment is preceded by a joining (J) gene segment; in addition, there are many cross-hybridizing variable (V) gene segments. To evaluate the extent of sequence diversity of L1 V regions, we screened three cDNA libraries, constructed from mitogen-stimulated Xenopus splenocytes, with probes for the C or the J gene segment. Eighteen cDNA clones that contain complete or truncated V regions were chosen for sequence analysis. The C regions of all clones are identical or nearly identical to the genomic C gene segment; the V regions are greater than 80% identical in nucleotide sequence and are presumably derived from a single family of V gene segments. Although framework regions are nearly identical, complementarity-determining regions are quite diverse. The expressed J segments fall into distinct groups, suggesting the presence of more than one germ-line J segment. Therefore, a genomic library was screened with a J region probe. A clone overlapping with the previously identified J-C clone, and containing four additional J gene segments, was isolated. All five J gene segments are very similar and three are identical in nucleotide sequence. Each of the three distinct germ-line J sequences is represented in the set of cDNA clones, suggesting that combinatorial diversification occurs; imprecision of V-J joining also appears to contribute to variability. Overall, these results suggest that the immunoglobulin repertoire in this species is not significantly restricted by a limitation in the diversity of light chain V regions.