Fig. 5.
Proposed mechanism of TPP-mediated alternative splicing of the C. reinhardtii THIC transcript. (A) Diagram of the sixth intron of the THIC gene, which contains the riboswitch, showing the intron boundaries (GU/AG), the splice acceptor sites (A), and the alternatively spliced exon in red. The green lines show which regions would be joined to form lariat loops. The thick lines depict regions that can form stems. At high TPP concentrations, TPP will be bound to the riboswitch, which means that base pairing occurs between P1 and P1′. Within the intron, there are then two splice acceptor sites available, so that two splicing events occur on either side of the riboswitch. This produces the THICL transcript that encodes a prematurely truncated protein. (B) When there is no TPP bound to the riboswitch (as might be predicted at low intracellular TPP concentrations), it is possible for alternative base pairing to form between P1′ and the region containing the 5′ splice acceptor site (P1″), which is colored blue. There is now only one splice acceptor site available at the 3′ end of the intron, and so the entire intron is removed to form the functional THICS transcript. The nucleotide sequence in Fig. 2C corresponds to the sequence from P1′ to P1.