Structure and expression of the pulmonary surfactant protein SP-C gene in the mouse

Genomic and cDNA clones encoding the murine pulmonary surfactant protein SP-C were isolated and sequenced to provide the primary amino acid sequence of the murine SP-C polypeptide and the structural organization of the SP-C gene locus. Murine SP-C is encoded by a single locus spanning approximately 3.2 kilobases of genomic DNA. The gene is composed of six exons and five introns, and its structure is closely related to the human SP-C gene. The primary transcript for the murine SP-C preprotein of Mr = 21,000 has an overall nucleotide identity of 80% with the coding sequence of the human precursor. The active airway peptide of 33-35 amino acids was even more closely related to the human SP-C airway peptide, sharing 95% identity at the amino acid level. The most significant structural difference between the murine and human SP-C genes was an increased size of the first intron in the murine SP-C gene. A single mRNA encoding murine SP-C of 0.8 kilobase was detected only in lung tissue, and SP-C mRNA was more abundant in neonatal and postnatal lung tissue than in fetal lung tissue. The nucleotide sequence of the promoter and 5'-flanking regions of the murine SP-C gene shared considerable identity with the human SP-C gene. Transgenic mice bearing a chimeric gene composed of the 5'-flanking and promoter regions of the human SP-C gene and the bacterial chloramphenicol acetyltransferase reporter gene were generated, demonstrating that lung-specific and developmental expression was conferred by these 5'-sequences. The chimeric gene was selectively expressed in preparations of respiratory epithelial cells, but was not expressed in alveolar macrophages isolated from the transgenic mice. The shared structural organization and developmental and tissue-specific expression of the SP-C gene between the mouse and human demonstrate considerable phylogenetic conservation of this gene and protein.