The mouse immunoglobulin kappa light-chain genes are located in early- and late-replicating regions of chromosome 6

The murine immunoglobulin kappa (κ) light-chain multigene family includes the constant region (Cκ), joining-region genes, and approximately 30 kappa-variable (Vκ) region families. The entire region occupies an estimated 1,000 to 3,000 kilobases, and some Vκ families have been linked by recombinant inbred mapping. The Cκ gene and 14 Vκ families replicated differently among cell lines of lymphoid and nonlymphoid origin. In nonlymphoid cells, the Cκ gene replicated earlier than the Vκ families. A transition from replication during the second third of S phase for the Cκ gene to later replication during S for Vκ families was observed. The Vκ family (Vκ21) that maps closest to the Cκ gene, replicated during the first half of the S phase; most of the other Vκ families replicated during the second half of S, and some replicated during the last quarter of the S phase. In lymphoid cells, the κ locus replicated earlier in the pre-B than in the B-cell lines. In one pre-B-cell line, 22D6, the κ genes examined replicated at the beginning of the S phase. In the B-cell lines, the EcoRI segment containing the transcribed gene replicated near the beginning of the S phase. Other Vκ families replicated within the first two-thirds of S phase. Some linked Vκ families replicated at similar times. In the B-cell lines, a transition from replication at the beginning of S for the transcribed Cκ and Vκ genes and surrounding DNA sequences to later replication for the other Vκ families was observed. However, in contrast to the non-lymphoid cell lines, the replication of this locus occurred predominantly during the first half of S. The κ locus contains both early- and late-replicating genes, and early replication is usually associated with transcriptional activity. The results are discussed with respect to the organization of transcriptionally active chromatin domains.