Comparison of mRNA precursors in plasmacytomas producing closely related kappa chains

To help elucidate the mechanisms of formation and expression of active immunoglobulin genes, we have made a systematic study of the processing patterns of the mRNAs that code for a closely related family of kappa chains. Among the members of this family, represented by the V(kappa)21 group of plasmacytomas, are examples in which six different germ-line V genes are joined to four different J segments in various combinations. The mRNA precursors were identified by hybridizing a cloned kappa-cDNA probe to poly(A)-containing nuclear RNAs that were size fractionated on methylmercury-agarose gels and transferred to diazotized paper. Based on the length of the segment excised in the last detectable processing step, which presumably represents the removal of the J-C intron, the precursor patterns were classified into four primary categories that correlated well with the type of J segment being expressed. The J segments were thus located at distances ranging from 2.4 to 4.8 kilobases from the constant-region gene. Different V genes joined to the same J segment exhibited similar precursor patterns, suggesting that the size of the translocated V segment may be relatively uniform among members of the V(kappa)21 group. A large (9.1 kilobases) component, which is likely to represent a primary transcription product, was observed in all processing categories, regardless of which J segment was being utilized. This surprising observation leads to some interesting predictions about the mechanism of V-J translocation.