Length heterogeneity in rat salivary gland alpha 2 mu globulin mRNAs: multiple splice-acceptors and polyadenylation sites.

Rat alpha 2 mu globulins are coded for by a family of about 25 structurally related genes, some of which are expressed in the male adult liver while the other subset seems to be active in several excretory organs, including salivary and lacrymal glands. To estimate the number and specificity of genes expressed in the salivary glands, we determined nucleotide sequences of 30 cDNA clones. At least two alpha 2 mu globulin genes are active and two thirds of mRNAs were shown to code for the peptide two amino acids shorter than the others. Unexpected observation was the intense length polymorphism in the 3' non-coding 6th intron-7th exon regions presumably caused by alternative splice-acceptor selection. At least six acceptor sites were utilized and the longest type retained the entire 6th intronic sequence resulting in a formation of unusually longer product. A stable mRNA molecule of this type was demonstrated in salivary glands by Northern blotting probed with the 6th intron-specific fragment. Together with three independent polyadenylation sites, the rat salivary glands generate a diverse set of alpha 2 mu globulin mRNAs.