Differing expression patterns and evolution of the rat kininogen gene family.

The present investigation using molecular cloning and sequence analysis concerns the examination of the molecular basis for different expression patterns of two types of the rat kininogen genes. We show that the low molecular weight and high molecular weight forms of K kininogens are produced from a single gene through alternative usage of two 3'-coding regions, whereas only the low molecular weight forms of T kininogens are generated as a result of several mutational changes in the high molecular weight-specifying regions of both T-I and T-II kininogen genes. The mutational changes include a nucleotide substitution at the polyadenylation/processing signal site, nucleotide deletions resulting in the frame-shift mutation, and an insertion of the type 2 Alu-equivalent sequence. Because kininogens represent a multifunctional protein comprising the proteinase-inhibitory activity, the kinin moiety, and the clotting activity, these results present evidence indicating the molecular basis for the disappearance of a part of the gene functions. We also show that the K and T kininogen genes as well as the two T kininogen genes are extremely homologous, excluding and including the above mutational changes, respectively. These structural relationships allow us to envisage evolutionary processes for the generation of the rat kininogen gene family, particularly for the disappearance of a part of the gene functions.