Endohyphal bacteria (EHB), dwelling within fungal hyphae, markedly affect the growth and metabolic potential of their hosts. To date, two EHB belonging to the family Burkholderiaceae have been isolated and characterized as new taxa, Burkholderia rhizoxinica (HKI 454T) and Mycoavidus cysteinexigens (B1-EBT), in Japan. Metagenome sequencing was recently reported for Mortierella elongata AG77 together with its endosymbiont M. cysteinexigens (Mc-AG77) from a soil/litter sample in the USA. In the present study, we elucidated the complete genome sequence of B1-EBT and compared it with those of Mc-AG77 and HKI 454T. The genomes of B1-EBT and Mc-AG77 contained a higher level of prophage sequences and were markedly smaller than that of HKI 454T. Although the B1-EBT and Mc-AG77 genomes lacked the chitinolytic enzyme genes responsible for invasion into fungal cells, they contained several predicted toxin-antitoxin systems including an insecticidal toxin complex and PIN domain imposing an addiction-like mechanism essential for endohyphal growth control during host colonization. Despite the different host fungi, the alignment of amino acid sequences showed that the HKI 454T genome consisted of 1,265 (32.6%) and 1,221 (31.5%) orthologous coding sequences (CDSs) with those of B1-EBT and Mc-AG77, respectively. This comparative study of three phylogenetically associated endosymbionts has provided insights into their origin and evolution, and suggests the later bacterial invasion and adaptation of B1-EBT to its host metabolism.
A key mechanism of signal transduction in eukaryotes is reversible protein phosphorylation, mediated through protein kinases and protein phosphatases (PPases). Modulation of signal transduction by this means regulates many biological processes. Saccharomyces cerevisiae has 40 PPases, including seven protein phosphatase 2C (PP2C PPase) genes (PTC1-PTC7). However, their precise functions remain poorly understood. To elucidate their cellular functions and to identify those that are redundant, we constructed 127 strains with deletions of all possible combinations of the seven PP2C PPase genes. All 127 disruptants were viable under nutrient-rich conditions, demonstrating that none of the combinations induced synthetic lethality under these conditions. However, several combinations exhibited novel phenotypes, e.g. the Δptc5Δptc7 double disruptant and the Δptc2Δptc3Δptc5Δptc7 quadruple disruptant exhibited low (13°C) and high (37°C) temperature-sensitive growth, respectively. Interestingly, the septuple disruptant Δptc1Δptc2Δptc3Δptc4Δptc5Δptc6Δptc7 showed an essentially normal growth phenotype at 37°C. The Δptc2Δptc3Δptc5Δptc7 quadruple disruptant was sensitive to LiCl (0.4 M). Two double disruptants, Δptc1Δptc2 and Δptc1Δptc4, displayed slow growth and Δptc1Δptc2Δptc4 could not grow on medium containing 1.5 M NaCl. The Δptc1Δptc6 double disruptant showed increased sensitivity to caffeine, congo red and calcofluor white compared to each single deletion. Our observations indicate that S. cerevisiae PP2C PPases have a shared and important role in responses to environmental stresses. These disruptants also provide a means for exploring the molecular mechanisms of redundant PTC gene functions under defined conditions.
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