The requirement of the METE isoform of methionine synthase for vitamin B12 independence in Chlamydomonas reinhardtii

K.E. Helliwell1, S. Sasso1, M. Croft1, A. Molnar1, G.L. Wheeler2, A.G. Smith1

1Plant Metabolism Group, Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EA, United Kingdom 2 Plymouth Marine Laboratories, Prospect Place, The Hoe, Plymouth PL1 3DH, UK

Algae cannot synthesise vitamin B12 and acquire this co-factor through a symbiotic relationship with bacteria. Vitamin B12 auxotrophy occurs throughout the algal kingdom and is not restricted to any one algal lineage. The isoform of methionine synthase possessed by the alga may act as a determining factor for this varying dependence. Vitamin B12 is an essential cofactor for the vitamin B12 dependent isoform (METH), although a second isoform (METE) functions independently of vitamin B12. Chlamydomonas reinhardtii encodes both the METE and METH genes and so it is capable of facultative B12 independence according to B12 levels. METH is expressed both in the presence and absence of vitamin B12, whereas METE is expressed only in the absence of the vitamin. This is similar to the situation in Escherichia coli where METH is used in preference in the presence of B12 as it has a higher catalytic activity. Here we examine the effects of the knock down of METE expression in C. reinhardtii using an RNA silencing approach. The growth of these knockdown lines have been assessed and compared to Lobomonas rostrata, a close relative of C. reinhardtii that contains only the METH gene and is thus dependent upon vitamin B12 for growth. The effect of the METE knockdown on METH expression is being investigated. Sequence similarity searches have been also used to identify the methionine synthase isoform/s present in newly sequenced algal genomes.

e-mail address of presenting author: keh53@cam.ac.uk