Alternative transcriptional initiation and splicing of mouse Lamc2 message

To investigate the mechanism of two forms of messages (5.1 and 2.4 kb) in mouse Lamc2, a gene encoding for the gamma2 chain of epithelial cell-specific laminin 5, we analyzed approximately 40 kb of genomic DNA containing the sequences from the first intron to the 23rd exon to the 3' untranscribed region. With the 5' rapid amplification of cDNA end (RACE) and primer extension technique using RNA from mouse kidney and thymic epithelial cells, we found that the smaller message starts at the 2895th base of the cDNA within the 19th exon. Upstream sequences of this alternative start site showed the promoter activity in the reporter assay with the secreted form of alkaline phosphatase (SEAP). We also identified a novel alternatively spliced exon (exon 19B) that includes two stop codons. These results revealed two possible additional open reading frame that are different from the previously described alternative human gamma2 peptide. Therefore, the mechanism for generating smaller message and the event of alternative splicing are quite distinct between mouse and human Lamc2, although genomic organization is highly conserved. The significance of the presence of alternative message is discussed.