Molecular analysis of gene expression in mouse salivary glands.

The synthesis of mouse salivary gland proteins was studied, in particular that of renin, an enzyme normally made in the kidney and involved in the maintenance of normal blood pressure, which is produced in the submaxillary gland (SMG) in high levels. This gene expression is both androgen inducible and genetically controlled, inbred strains of mice having either high or low levels of the enzyme. It was found that the differences in renin levels reported by other workers correlated with a variation in translatable mRNA levels. The isolation and identification of cDNA clones corresponding to SMG renin facilitated the identification of two renin genes in the DBA/2 mouse (a high producer strain). Primer- extension analysis and S1 mapping using a restriction fragment from one of these genes, Ren-1, revealed three potential start points for renin transcription. In the high producer all three appear to be used but in the low producer (C57BL/10) only one is significantly active (P1). Renin cDNA clones were also used to purify renin mRNA which was translated in-vitro. Subsequent processing of the primary translation product confirmed the presence of a leader sequence, completing the biosynthetic pathway of mouse SMG renin. The translation profile of mRNA from the sublingual gland (SL) showed an abundant 16kd polypeptide and reverse transcription of this mRNA indicated a single major size-class of mRNA of 600 nucleotides, sufficient to encode a polypeptide of such size. In-vitro translation and cDNA cloning studies using M13 as a vector have shown this major product to be a previously unrecognised family of polypeptides which appear to be constitutively expressed. The availability of renin sequence probes permitted the cloning and isolation of cDNA copies of human renal renin mRNA. The encoded polypeptide had a high degree of homology to mouse SMG and kidney renins but had three additional internal amino acids. The region corresponding to the cleavage site between the A and B chains of mouse renin is altered, giving support to reports that human renin is a single-chain molecule.