Alternative Splicing of the Last Intron Yields Transcripts Differentially Expressed in Human Tissues That Code In Vitro for a Protein Devoid of Triokinase and FMN Cyclase Activity.

The 18-exon human gene codes for dual-activity triokinase and FMN cyclase (TKFC) in an ORF, spanning from exon 2 to exon 18. In addition to TKFC-coding transcripts (classified as type by their intron-17 splice), databases contain evidence for alternative transcripts, but none of them has been expressed, studied, and reported in the literature. A novel full-ORF transcript was cloned from brain cDNA and sequenced (accession no. DQ344550). It results from an alternative 3' splice-site in intron 17. The cloned cDNA contains an ORF also spanning from exon 2 to exon 18 of the gene but with a 56-nt insertion between exons 17 and 18 (classified as type). This insertion introduces an in-frame stop, and the resulting ORF codes for a shorter TKFC variant, which, after expression, is enzymatically inactive. intron-17 splicing was found to be differentially expressed in human tissues. In a multiple-tissue northern blot using oligonucleotide probes, the liver showed a strong expression of the -like splice of intron 17, and the heart preferentially expressed the -like splice. Through a comparison to global expression data from massive-expression studies of human tissues, it was inferred that the intestine preferentially expresses transcripts that contain neither of those splices. An analysis of transcript levels quantified by RNA-Seq in the GTEX database revealed an exception to this picture due to the occurrence of a non-coding short transcript with a -like splice. Altogether, the results support the occurrence of potentially relevant transcript variants of the gene, differentially expressed in human tissues. (This work is dedicated in memoriam to Professor Antonio Sillero, 1938-2024, for his lifelong mentoring and his pioneering work on triokinase).