The Gram-negative bacterium Candidatus Glomeribacter gigasporarum is an obligate endobacterium living inside the cytoplasm of the arbuscular mycorrhizal fungus Gigaspora margarita. We sequenced its genome, leading to a 1.8-Mb final assembly. Phylogenetic analyses placed Ca. G. gigasporarum among Burkholderiaceae, whereas metabolic network analyses clustered it with insect endobacteria, revealing that it has undergone convergent evolution to adapt to an intracellular lifestyle. Ca. G. gigasporarum has an extreme dependence on its host for nutrients and energy, whereas the fungal host is itself an obligate biotroph that relies on a photosynthetic plant. It is thus surprising that, notwithstanding its reduced genome, the endobacterium possesses genes related to the production of secondary metabolites. It retains indeed the whole biosynthetic pathway for vitamin B12, as well as two large genes coding for nonribosomal peptide synthase and polyketide syntase. These features may contribute to the fungal host"s ecological fitness. Ca. G. gigasporarum proliferates inside a fungal phagosome/vacuole, suggesting that its communication with the fungal host should be mediated by some, still unknown, molecular determinants. A few sequences that are likely to code for putative secreted effectors were identified, and their potential role in regulating the bacterial-fungal interaction is being to be further investigated. Lastly, many toxin-antitoxin (TA) modules were found in the Ca. G. gigasporarum genome, and their expression was monitored over the fungal life cycles' stages. For one of them the toxic activity was demonstrated heterologously. These data open the question whether TA systems might play a role in the functioning of such fungal-bacterial symbiosis.
The genome of the obligate endobacterium of an AM fungus reveals an interphylum network of signal and nutrient exchanges
Ghignone S;
2016
Abstract
The Gram-negative bacterium Candidatus Glomeribacter gigasporarum is an obligate endobacterium living inside the cytoplasm of the arbuscular mycorrhizal fungus Gigaspora margarita. We sequenced its genome, leading to a 1.8-Mb final assembly. Phylogenetic analyses placed Ca. G. gigasporarum among Burkholderiaceae, whereas metabolic network analyses clustered it with insect endobacteria, revealing that it has undergone convergent evolution to adapt to an intracellular lifestyle. Ca. G. gigasporarum has an extreme dependence on its host for nutrients and energy, whereas the fungal host is itself an obligate biotroph that relies on a photosynthetic plant. It is thus surprising that, notwithstanding its reduced genome, the endobacterium possesses genes related to the production of secondary metabolites. It retains indeed the whole biosynthetic pathway for vitamin B12, as well as two large genes coding for nonribosomal peptide synthase and polyketide syntase. These features may contribute to the fungal host"s ecological fitness. Ca. G. gigasporarum proliferates inside a fungal phagosome/vacuole, suggesting that its communication with the fungal host should be mediated by some, still unknown, molecular determinants. A few sequences that are likely to code for putative secreted effectors were identified, and their potential role in regulating the bacterial-fungal interaction is being to be further investigated. Lastly, many toxin-antitoxin (TA) modules were found in the Ca. G. gigasporarum genome, and their expression was monitored over the fungal life cycles' stages. For one of them the toxic activity was demonstrated heterologously. These data open the question whether TA systems might play a role in the functioning of such fungal-bacterial symbiosis.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
