Methionine synthase isoform is a key determinant in the evolution of vitamin B12 auxotrophy in algae
K. E. Helliwell1, K. Leptos2, R. Goldstein2, G. Wheeler3, and A.G. Smith1
1) Dept of Plant Sciences, University of Cambridge, Cambridge, CB2 3EA, UK
2) DAMTP, U. Cambridge, Cambridge, CB3 0WA, UK
3) Plymouth Marine Lab, Plymouth, PL1 3DH, UK
Many different algae require vitamin B12 for growth, but there is no consistent phylogenetic relationship between those that require the vitamin and those that do not, with even closely-related species differing in their requirements. This variability in cofactor dependence raises several interesting questions relating to the evolution of B12 dependence in algae. Vitamin B12 is an essential cofactor for the vitamin B12 dependent isoform of methionine synthase (METH). However, a second isoform (METE) functions independently of vitamin B12. Chlamydomonas reinhardtii possesses both the METE and METH genes and does not require B12. Also METE expression is repressed in the presence of vitamin B12 in C. reinhardtii (Croft et al. 2005). With the completion of several algal genome projects, a wealth of genomic information is now available for representatives of key phylogenetic groups. Using this resource, alongside studies of physiology and gene regulation, we have accumulated evidence that points to multiple losses of vitamin B12-independent methionine synthase (METE) as being a key factor in the evolution of B12 dependence in algae. Evidence in support of this theory includes the identification of a METE unitary pseudogene in Volvox carteri, a close relative of C. reinhardtii, which suggests that B12 dependence evolved relatively recently in this lineage. Moreover, the observation that METE expression is also repressed by vitamin B12 in a marine diatom, which is phylogenetically distant from C. reinhardtii, provides a mechanism for gene loss. Given that eukaryotes must obtain vitamin B12 from prokaryotes, the selective loss of B12-independent methionine synthase in many different algae may have had important physiological and ecological consequences for these lineages.
e-mail address of presenting author: