Genetics of Magnetosome Formation
Over the last decade a combination of approaches by various groups have led to the identification of a "core" set of magnetosome formation genes. Interestingly, these genes are organized into a genomic island in all magnetotactic bacteria. Using genetic approaches we have delineated the putative function of a large number of these genes and are now looking at their specific functions. In particular, we have shown that the genes within the mamAB gene cluster regulate nearly every step of magnetosome formation (Murat et al. PNAS 2010). In fact, in our recent work (and published work from the Schüler group) we have shown that this cluster is also sufficient for magnetosome formation. Current projects in the lab include further genetic analysis of magnetosome formation in M. magneticum AMB-1 and other magnetotactic bacteria. We are interested in defining specific genetic modules that control various aspects of organelle formation. Advances in this area can be directly relayed into applications where magnetosome genes will be transfered to other organisms.
One of our motivations for the study of magnetosome formation is to gain an understanding of general cell biological processes in bacteria. This idea has been validated in our work on magnetosome chain organization. In collaboration with Grant Jensen's group, we showed that magnetosome chains were surrounded by cytoskeletal filaments likely composed of the actin-like protein MamK. MamK is encoded by the mamAB gene cluster and its deletion results in the misorganization of the magnetosome chain (see image to the left and Komeili, Li et al. Science 2006). Actin-like proteins are nearly ubiquitous in the bacterial world and, unlike their eukaryotic relatives, form numerous highly divergent families that seem to have been specialized for distinct functions. The MreB family, for example, plays a central role in organizing the bacterial cell wall synthesis machinery, and other families, such as ParM and AlfA, are required for plasmid segregation. MamK defines a unique family of the bacterial actins and can in fact be found in non-magnetotactic bacteria. Thus, we are interested not only in the function of MamK in magnetotactic bacteria but also its properties as a cytoskeletal protein. Recently, we have shown that MamK forms dynamic filaments in vivo that are regulated by the redundant action of mamJ and limJ, two genes found within the magnetosome island (Draper, Byrne et al. Mol Micro 2011). We are now attempting to reconstitute MamK filament dynamics in vitro and isolate its interaction partners.
Ever since their discovery nearly four decades ago, magnetotactic bacteria have been hailed as a potentially powerful system to study the molecular mechanisms of biomineralization. Biomineralization processes are widespread in nature, and magnetotactic bacteria are the only bacterial system capable of directing the intracellular precipitation of nanometer-sized inorganic crystals. Thus, it has been speculated that the magnetosome may be the ancestral, or at least the most ancient, biomineralization system. Over the last few years, my group’s global approach to understanding the process of magnetosome formation has provided us with unique opportunities to gain new insights into the process of biomineralization. We have isolated numerous mutants where the size and shape of magnetic minerals are severely altered. For instance, we have shown that the putative protease activity of MamE (yet another mamAB cluster-encoded protein) is essential for the maturation of magnetite crystals (Quinlan et al. Mol Micro. 2011). We have also found that mmsF (also known as amb0957) is a major regulator of magnetite biomineralization (see figure above). We are now interested in defining the genetic and physical interplay between these factors in order to define the specific mechanisms of biomineralization in more detail.
Diversity of Magnetosome Formation
Magnetotactic bacteria are incredibly diverse and display a stunning array of variation in the quantity and appearance of their magnetosome chains. We are interested in understanding the basis of this phenotypic diversity by developing other magnetotactic bacteria for genetic and molecular work. In particular, we are working with Desulfovibrio magneticus RS-1, a delta-proteobacterium that produces tooth-shaped crystals as opposed to the cubo-octahedral crystals of AMB-1. While RS-1 contains a highly edited version of the magnetosome island, it is missing many of the proteins thought to shape magnetite crystals in the magnetospirilla. So does RS-1 have a unique mode of biomineralization? Recently, we showed that RS-1 has a distinct pattern to its biomineralization kinetics and displays a highly intricate ultrastructure. In fact, we found that RS-1 contains a second iron-containing organelle distinct from its magnetosomes (see Byrne et al. PNAS. 2010 and video of RS-1 ultrastructure). These findings raise a number of interesting questions regarding the molecular mechanisms of biomineralization in RS-1. How is crystal shape determined in this organism? Are the functions of magnetosome proteins conserved in RS-1? What is the function of the other iron-containing organelle in RS-1?