Unraveling Vitamin B₁₂-Responsive Gene Regulation in Algae

• Published in: Plant physiology (Bethesda) Vol. 165; no. 1; pp. 388 - 397
• Main Authors: Helliwell, Katherine E., Scaife, Mark A., Sasso, Severin, Araujo, Ana Paula Ulian, Purton, Saul, Smith, Alison G.
• Format: Journal Article
• Language: English
• Published: United States American Society of Plant Biologists 01.05.2014
• Subjects: 5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase - chemistry; 5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase - genetics; 5-Methyltetrahydrofolate-Homocysteine S-Methyltransferase - metabolism; Algae; Amino Acid Sequence; Bacteria; BIOCHEMISTRY AND METABOLISM; Chlamydomonas - drug effects; Chlamydomonas - genetics; Gels; Gene expression; Gene expression regulation; Gene Expression Regulation, Plant - drug effects; Genes; Genes, Reporter; Microalgae - drug effects; Microalgae - enzymology; Microalgae - genetics; Molecular Sequence Data; Proteomics; Repression; Response Elements - genetics; Untranslated regions; Vitamin B 12 - pharmacology; Vitamin B12
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.113.234369

Photosynthetic microalgae play a vital role in primary productivity and biogeochemical cycling in both marine and freshwater systems across the globe. However, the growth of these cosmopolitan organisms depends on the bioavailability of nutrients such as vitamins. Approximately one-half of all microalgal species requires vitamin B₁₂ as a growth supplement. The major determinant of algal B₁₂ requirements is defined by the isoform of methionine synthase possessed by an alga, such that the presence of the B₁₂-independent methionine synthase (METE) enables growth without this vitamin. Moreover, the widespread but phylogenetically unrelated distribution of B₁₂ auxotrophy across the algal lineages suggests that the METE gene has been lost multiple times in evolution. Given that METE expression is repressed by the presence of B₁₂, prolonged repression by a reliable source of the vitamin could lead to the accumulation of mutations and eventually gene loss. Here, we probe METE gene regulation by B₁₂ and methionine/folate cycle metabolites in both marine and freshwater microalgal species. In addition, we identify a B₁₂-responsive element of Chlamydomonas reinhardtii METE using a reporter gene approach. We show that complete repression of the reporter occurs via a region spanning -574 to -90 bp upstream of the METE start codon. A proteomics study reveals that two other genes (S-Adenosylhomocysteine hydrolase and Serine hydroxymethyltransferase2) involved in the methionine-folate cycle are also repressed by B₁₂ in C. reinhardtii. The strong repressible nature and high sensitivity of the B₁₂-responsive element has promising biotechnological applications as a cost-effective regulatory gene expression tool.