A genetic strategy for probing the functional diversity of magnetosome formation.

Lilah Rahn-Lee,Timothy Milbourne,Ronald L Walsworth,Manjing Zhang,David R Glenn,Meghan E Byrne,Hojatollah Vali,Arash Komeili,David Le Sage,Daniel B Kearns

doi:10.1371/journal.pgen.1004811

Abstract

Model genetic systems are invaluable, but limit us to understanding only a few organisms in detail, missing the variations in biological processes that are performed by related organisms. One such diverse process is the formation of magnetosome organelles by magnetotactic bacteria. Studies of model magnetotactic α-proteobacteria have demonstrated that magnetosomes are cubo-octahedral magnetite crystals that are synthesized within pre-existing membrane compartments derived from the inner membrane and orchestrated by a specific set of genes encoded within a genomic island. However, this model cannot explain all magnetosome formation, which is phenotypically and genetically diverse. For example, Desulfovibrio magneticus RS-1, a δ-proteobacterium for which we lack genetic tools, produces tooth-shaped magnetite crystals that may or may not be encased by a membrane with a magnetosome gene island that diverges significantly from those of the α-proteobacteria. To probe the functional diversity of magnetosome formation, we used modern sequencing technology to identify hits in RS-1 mutated with UV or chemical mutagens. We isolated and characterized mutant alleles of 10 magnetosome genes in RS-1, 7 of which are not found in the α-proteobacterial models. These findings have implications for our understanding of magnetosome formation in general and demonstrate the feasibility of applying a modern genetic approach to an organism for which classic genetic tools are not available.

Highlights

Genetic analysis historically relied on model systems that were easy to manipulate and for which genetic maps, later full genome sequences, were available
Conjugation of AMB-1, an excellent genetic model system, yields 1023 transconjugates per recipient cell [19], which results in thousands of transposon insertions per conjugation and allows for efficient screening of the entire genome
RS-1 is well placed to expand our understanding of magnetosome synthesis because it is a representative of a larger group of d-proteobacteria, which produce bullet-shaped magnetite crystals

Summary

Introduction

Genetic analysis historically relied on model systems that were easy to manipulate and for which genetic maps, later full genome sequences, were available. Today many interesting organisms have their genomes sequenced, but cannot be manipulated due to their incompatibility with standard genetic tools. One such organism is Desulfovibrio magneticus RS-1 (RS-1). It was hypothesized that this behavior allows the bacteria, which were isolated in the northern hemisphere, to swim northwards to the bottom of the water column where micro-oxic or anoxic conditions exist. This behavior is due to intracellular singledomain magnetite or greigite crystals organized in one or more chains along the length of the cell [4]. Magnetotactic bacteria have inspired studies in bacterial cell biology [5], biomineralization [6], and nanotechnology [7]

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Results

Conclusion