Heterogeneous Sp1 mRNAs in human HepG2 cells include a product of homotypic trans-splicing

Spl is one of the well documented transcription factors, but the whole structure of human Spl has not been determined yet. In the present study, we isolated several cDNAs representing two forms of human Spl mRNA with different 5'-terminal structures in HepG2 cells. Isolation of a genomic clone established that one of the cDNAs represents the mRNA having consecutive alignment of exons, which allowed deducing the complete amino acid sequence for human Spl. Another cDNA clone had a surprising structure that possessed an alignment of exons 3-2-3. Both reverse transcriptase-polymerase chain reaction and RNase protection assays confirmed accumulation of the two forms of Spl mRNA in HepG2 cells. Because Southern blot analysis suggested that exon 3 is of a single copy in the genome, the cDNA clone having the duplicated sequences for exon 3 appeared to reflect the trans-splicing between pre-mRNAs of human Sp1.