The 39 untranslated region (UTR) of ThiC, encoding the enzyme
thiamine synthase C, contains a TPP-regulated riboswitch. When
TPP levels are high, TPP binds to the 39 UTR and alternative
splicing of the pre-mRNA encoding the ThiC protein occurs. This
results in the production of three different transcripts: the coding
sequence (ThiC1), intron retention variant (ThiC2) (increases when
TPP is low), and an intron splice variant (ThiC3) (decreases when
TPP is low) (Wachter et al., 2007). The tissue used in the experiments
in Figures 11 and 12 was taken from seedlings within the
ST1 development stage as changes in TPP levels were most
pronounced at this point. In all three TKox lines, the abundance of
the ThiC2 splice variant was higher than wild-type plants, while that
of ThiC3 transcript was decreased substantially and consistently in
cotyledons from both 3- and 9-d-old seedlings. Taken together,
these results suggest that the pattern of expression of the ThiC
alternative transcripts in the TKox lines is consistent with a low TPP
phenotype (Figure 11)