| Popis: |
Vitamin B12-dependent algae are unable to grow in medium made with natural filter-sterilized seawater or freshwater unless a vitamin B12 supplement is added to the culture, suggesting that the concentration of free vitamin B12 in the natural environment in generally too low to support the growth of vitamin B12-dependent algae. The source of cobalamin for algae appears to be vitamin B12-synthesizing bacteria, because when these organisms are grown in co-culture, the algae no longer require the vitamin. In return for vitamin B12, the bacteria are able to use the products of algal photosynthesis to grow, indicating an important and unsuspected symbiosis. Using the green alga Chlamydomonas reinhardtii as a model organism, RT-PCR analysis has demonstrated that vitamin B12 regulates the expression of the gene encoding the vitamin B12-independent methionine synthase. Cobalamin is likely to regulate the expression of this gene by binding directly to the mRNA and influencing its secondary structure, since affinity binding studies suggest that the mRNAs that encode for the methionine synthase proteins have a high affinity for cobalamin. Further work in C. reinhardtii and Euglena gracilis has shown that a cell wall protein is involved in the high affinity uptake of vitamin B12, since mutants deficient in cell wall biosynthesis are impaired in their ability to take up the vitamin. The C. reinhardtii protein has been purified by affinity chromatography, and identified by mass spectrometry. The complete cDNA encoding the C. reinhardtii vitamin B12 binding protein has been cloned, and the protein shown to be a member of the pherophorin family, a well-known group of green algal cell wall proteins. A combination of techniques has indicated that the protein is glycosylated, and that the sugar moieties are attached to a central domain that contains a significant number of hydroxyproline residues. |
