PH-115:

Complete genome sequence of a novel endobacterium Mycophilus cysteinexigens from soil fungus Mortierella elongata FMR23-6

Endohyphal bacteria harboring fungi play a role in the environmental adaptation ability of host fungi. In our previous study, a fungal endobacterium belonging to the family Burkholderiaceae was isolated from inside the mycelium of soil fungus Mortierella elongata FMR23-6. The isolated endobacterium (strain B1-EB) represented a novel species in a novel genus, for which Mycophilus cysteinexigens, gen. nov., sp. nov. is proposed. Its genome DNA was sequenced completely to date. In this study, the B1-EB genome sequence was compared with a genome sequence of the rhizoxin-producing endobacterium, Burkholderia rhizoxinica HKI 454 harboring the phytopathogenic fungus Rhizopus microsporus, to characterize the mechanism of a symbiotic relationship between fungi and bacteria. The circular chromosome of B1-EB contains 2.8 Mb, for which 2,317 ORFs were predicted. There were 45 phage-related genes and 118 transposable gene fragments on the B1-EB genome, indicating that genome modification was repeated. In addition, the B1-EB genome lacks cysteine biosynthetic pathway, suggesting that the strain requires cysteine from the host fungus. The tcaABC-tccD gene cluster encoding the insecticidal toxin complex protein was found in the B1-EB genome, promoting us to postulate that the host fungus had acquired a defense system against antagonistic organism. Moreover, the genome contained three copies of type III secretion system gene cluster, which was reported to play a crucial role in bacterial infection into the fungal mycelia. The HKI 454 genome has the bacterial type II secretion system, which releases chitinolytic enzymes and chitin-binding proteins for the formation of the endosymbiosis. In contrast, these genes are not found in the B1-EB genome, suggesting that strain B1-EB lacks the invasion function to M. elongata. Our genome analysis revealed the interdependence between M. elongata and M. cysteinexigens and the endobacterial incomplete infection system.

keywords:genome sequence,endophytic bacteria,Burkholderiaceae,Mortierella elongata,Mycophilus cysteinexigens