Abstract

Vitamin B12 is an enzyme co-factor and acts on RNA regulatory elements called riboswitches to regulate gene expression. Our group showed that many algal species require vitamin B12 and that this requirement is met by co-culturing B12 dependent algae with bacteria that make B12 . Using the bacterium Mesorhizobium loti MAFF303099 in co-culture with a B12 dependent alga Lobomonas rostrata, stable growth of both species occurred. I investigated gene expression of bacteria in co-culture vs free-living state. The results indicated that there is some differential gene expression and that two factors are important – the time at which gene expression is quantified and the role of a putative riboswitch in regulating gene expression. As a result, I made promoter-reporter constructs to allow for repeated measurements of gene expression to capture the dynamic nature of nutrient exchange and growth on gene expression. In addition to this work, I have shown that different strains of M. loti such as R7A and a mutant lacking the symbiosis island called R7ANS are able to grow in co-culture with L. rostrata. This result indicates the possibility of wider vitamin sharing between the algae and bacteria. I am also making a vitamin B12 mutant to use in competition experiments where I will test the robustness of the mutualism to cheating strains of M. loti. Lastly, using various microscope techniques I am investigating the nature of the interaction between the alga and bacterium.