ORGANIZATION OF SERPIN GENE-1 FROM MANDUCA-SEXTA - EVOLUTION OF A FAMILY OF ALTERNATE EXONS ENCODING THE REACTIVE-SITE LOOP

Manduca sexta serpin gene-1 encodes a family of serpins whose amino ac id sequences are identical in their amino terminal 336 residues but va riable in their carboxyl-terminal 39-46 residues, which includes the r eactive site loop (Jiang, H., Wang, Y., and Kanost, M.R. (1994) J. Bio l. Chem, 269, 55-58). Here, we report the gene's complete nucleotide s equence and exon-intron structure. A unique characteristic of this gen e is its exon 9, which is present in 12 alternate forms between exons 8 and 10. Isolation and characterization of cDNA clones containing exo ns 9C, 9H, and 9I, which were not found previously, indicate that all 12 alternate forms of exon 9 can be utilized to generate 12 different serpins. The splicing pathway apparently allows inclusion of only one exon 9 per molecule of mature serpin-1 mRNA. Analysis of exon-intron b order sequences reveals unique features that may be involved in regula tion of RNA splicing. The exon 9 region has apparently evolved through rounds of exon duplication and sequence divergence. The exons near th e center of the region may have evolved recently, whereas the outermos t exons are the most ancient. Exons 9G and 9H were duplicated as a pai r from exons 9E and 9F, an event that may have occurred more than once in the history of this gene.