Accurate 5' splice-site selection in mouse kappa immunoglobulin light chain premessenger RNAs is not cell-type-specific.

In mature mouse B lymphocytes, immunoglobulin kappa light chain transcripts contain an intervening sequence separating the recombined variable (V) plus joining (J) exon from the distant constant (C) exon. After V-J recombination, this intervening sequence can include as many as three unused but very similar J-region 5' splice sites. Each of these sites is potentially functional if the gene is appropriately recombined. It is unclear how the splicing machinery distinguishes among these 5' splice sites, always choosing the most upstream site. We used synthetic transcripts of kappa gene sequences containing J3 and J4 in both the germ-line and the recombined configurations to study the pattern of 5' splice-site selection in vitro. We find that both HeLa cell and lymphocyte nuclear extracts fail to discriminate between the J3- and J4-region 5' splice sites. In contrast, after transfection into HeLa cells, similar kappa light chain transcripts are spliced correctly at the most upstream 5' splice site--that which is used in kappa-producing cells. We conclude that accurate 5' splice-site selection in the mouse kappa light chain is neither cell-type- nor species-specific. Potential mechanisms for this controlling step in gene expression are discussed.