In pre-B cells, immunoglobulin mu (Ig mu) is associated with pre-B cell-specific proteins to form a multimeric complex that is found on the cell surface. One of these proteins is encoded by the three exon Ig lambda-like gene 14.1, whose expression is restricted to pre-B cells and occurs from an unrearranged gene. A comparison of the 14.1 gene structure to the seven-gene human Ig lambda locus revealed that the most 5' gene, Ig lambda 1, is organized in a three-exon structure very similar to the 14.1 gene. Transcription and splicing of these three-exon sequences would lead to an mRNA with an open reading frame which could encode a light (L) chain-like protein with a molecular weight of 23,045. Our analysis suggests that two transcripts may be produced from the Ig lambda 1 gene that share the same Ig lambda 1 constant region-containing third exon. One transcript would include all three 14.1-related exons and be expressed from the germline gene, and the second transcript would be produced after variable-joining (V-J) recombination has occurred to Ig lambda J1 and would encode a classic Ig lambda L chain protein. The conservation of the genomic organization of the human 14.1 and Ig lambda 1 genes and the mouse homolog, lambda 5, relative to the classic Ig lambda L chain genes provides insight into the evolution of Ig genes.

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