Effect of PQQ glucose dehydrogenase overexpression in Escherichia coli on sugar-dependent respiration

Cultivation of a vampire: 'Candidatus Absconditicoccus praedator'

Halorhodospira halophila, one of the most-xerophilic halophiles, inhabits biophysically stressful and energetically expensive, salt-saturated alkaline brines. Here, we report an additional stress factor that is biotic: a diminutive Candidate-Phyla-Radiation bacterium, that we named 'Ca. Absconditicoccus praedator' M39-6, which predates H. halophila M39-5, an obligately photosynthetic, anaerobic purple-sulfur bacterium. We cultivated this association (isolated from the hypersaline alkaline Lake Hotontyn Nur, Mongolia) and characterized their biology. 'Ca. Absconditicoccus praedator' is the first stably cultivated species from the candidate class-level lineage Gracilibacteria (order-level lineage Absconditabacterales). Its closed-and-curated genome lacks genes for the glycolytic, pentose phosphate- and Entner-Doudoroff pathways which would generate energy/reducing equivalents and produce central carbon currencies. Therefore, 'Ca. Absconditicoccus praedator' is dependent on host-derived building blocks for nucleic acid-, protein-, and peptidoglycan synthesis. It shares traits with (the uncultured) 'Ca. Vampirococcus lugosii', which is also of the Gracilibacteria lineage. These are obligate parasitic lifestyle, feeding on photosynthetic anoxygenic Gammaproteobacteria, and absorption of host cytoplasm. Commonalities in their genomic composition and structure suggest that the entire Absconditabacterales lineage consists of predatory species which act to cull the populations of their respective host bacteria. Cultivation of vampire : host associations can shed light on unresolved aspects of their metabolism and ecosystem dynamics at life-limiting extremes.

Cloning and functional expression of glucose dehydrogenase complex of Burkholderia cepacia in Escherichia coli

The thermostable glucose dehydrogenase (GDH) from Burkholderia cepacia sp. SM4 is composed of a catalytic subunit (alpha), an electron transfer subunit (beta), and a small gamma subunit of unknown function. We cloned a 1428-nucleotide gene encoding the beta subunit located immediately downstream of the alpha subunit. This completes the isolation of the genes encoding the three components of the GDH complex, which are clustered very close together with the same transcription polarity in the order gammaalphabeta. The deduced beta subunit amino acid sequence contains three typical heme-binding motifs and was 44-49% identical to the cytochrome c subunits of other FAD-dependent dehydrogenase complexes. The GDHgammaalphabeta complex of B. cepacia was successfully expressed in a fully active form in Escherichia coli by co-expression with cytochrome c maturation genes. Recombinant expression of the GDH complex was also found to restore glucose-dependent respiration in a GDH mutant of E. coli.