L Chains Are Assembled by a Single Recombination Event

KEY CONCEPTS
- A λ chain is assembled through a single recombination event involving a V gene segment and a J_λ -C_λ gene segment.
- The V_λ gene segment has a leader exon, intron, and V_λ - coding region. The J_λ -C_λ gene segment has a short J_λ - coding exon, an intron, and a C_λ -coding region.
- A κ chain is assembled by a single recombination event involving a Vκ gene segment and one of five Jκ segments, all upstream of the Cκ gene.
A λ chain is assembled from two DNA segments (FIGURE 1). The V_λ gene segment consists of the leader exon (L) separated by a single intron from the V segment. The J_λ −C_λ gene segment consists of the J_λ segment separated by a single intron from the C_λ exon.
J is an abbreviation for “joining,” because the J segment identifies the region to which the V_λ segment becomes connected. Thus, the joining reaction does not directly involve V_λ and C_λ gene segments, but occurs via the J_λ segment (V_λ J_λ -C_λ joining). The J segment is short and codes for the last few amino acids of the variable region, as defined by amino acid sequence. In the complete gene generated by recombination, the V_λ -J_λ segment constitutes a single exon coding for the entire variable region.

FIGURE 1. The C_λ gene segment is preceded by a J_λ segment, so that V_λ -J_λ recombination generates a productive V_λ -J_λ C_λ .
A κ chain is also assembled from two DNA segments (FIGURE 2). However, the organization of the Cκ locus differs from that of the Cκ locus. A group of five Jκ segments is spread over a region of 500 to 700 bp, separated by an intron of 2 to 3 kb from the C exon. In the mouse, the central Jκ segment is nonfunctional (φJ3). A Vκ segment (which contains a leader exon, such as Vκ ) may be joined to any one of the Jκ segments. Whichever Jκ segment is used, it becomes the terminal part of the intact variable exon. Any Jκ segment upstream of the recombining Jκ segment is lost; any Jκ segment downstream of the recombining Jκ segment is treated as part of the intron between the V and C exons.

FIGURE 2.The Cκ gene segment is preceded by multiple J segments in the germline. Vκ -Jκ joining may recognize any one of the J segments, which is then spliced to the C gene segment during RNA processing.
All functional J segments possess a signal at their 5′ boundary that makes it possible to recombine with a V segment; they also possess a signal at the 3′ boundary that can be used for splicing to the C exon. Whichever J_L segment is recognized in DNA V-J_L joining, it will use its splicing signal in RNA processing.