Figure 2.

Evidence from Bertrand et al., 2012 that AdoMet SAM starvation is an important consequence of B₁₂ deprivation, with implications for thiamine biosynthesis. SAM synthetase (Tp 39946, Pt 18319) converts methionine and ATP to SAM. SAM, after use for methylation reactions, is converted to S-adenosylhomocysteine (SAH). SAH can act as an inhibitor to methylation reactions because of its high affinity for most methyltranserfases. SAH hydrolase (Tp 28496; Pt bd 913) catalyzes the reversible interconversion of SAH to homocysteine and adenosine. The expression of the genes encoding these proteins in two diatoms appears to correlate. RPKM (Reads Per Kilobase of exon model per Million mapped reads) gene expression values are plotted against each other for each of eight samples in two diatoms, duplicates of replete, low B₁₂, low B₁₂ with low iron, and low iron alone. Expression under iron limited conditions was examined along with B₁₂ to verify whether changes induced were likely a general stress response or more specific to the vitamin. In both diatoms, cells grown under nutrient replete conditions express these genes at the lowest level. Iron and B₁₂ availability both influence the expression of these genes, with B₁₂ having a greater impact of gene expression the B₁₂ requiring diatom T. pseudonana. ThiC is a SAM-dependent protein required for pyrimidine moiety synthesis in thiamine biosynthesis. The expression of genes encoding ThiC in both these diatoms is elevated under low B₁₂ availability and not under low iron availability, suggesting that thiamine biosynthesis, and B₁₂ availability may be linked in these diatoms, potentially through B₁₂ impacts on SAM availability.