Frankia irregularis sp nov., an actinobacterium unable to nodulate its original host, Casuarina equisetifolia, but effectively nodulates members of the actinorhizal Rhamnales

A red pigmented actinobacterium designated G2(T), forming extremely branched vegetative hyphae, vesicles and mutilocular sporangia, was isolated from Casuarina equisetifolia nodules. The strain failed to nodulate its original host plant but effectively nodulated members of actinorhizal Rhamnales. The taxonomic position of G2(T) was determined using a polyphasic approach. The peptidoglycan of the strain contained meso-diaminopimelic acid as diagnostic diamino acid, galactose, glucose, mannose, rhamnose, ribose and xylose. The polar lipid pattern consisted of phosphatidylinositol (PI), diphosphatidylglycerol (DPG), glycophospholipids (GPL1-2), phosphatidylglycerol (PG), aminophospholipid (APL) and unknown lipids (L). The predominant menaquinones were MK-9 (H-4) and MK-9 (H-6) while the major fatty acids were iso-C-16 : 0, C-17 : 1 omega 8c and C-15 : 0. The size of the genome of G2(T) was 9.5 Mb and digital DNA G+C content was 70.9 %. The 16S rRNA gene showed 97.4-99.5% sequence identity with the type strains of species of the genus Frankia. Digital DNA-DNA hybridisation (dDDH) values between G2(T) and its nearest phylogenetic neighbours Frankia elaeagni and Frankia discariae were below the threshold of 70 %. On the basis of these results, strain G2(T) (=DSM 45899(T)=CECT 9038(T)) is proposed to represent the type strain of a novel species Frankia irregularis sp. nov.