What are the stories that the Frankia genomes are telling us?

TISA L; BEAUCHEMIN N; BOGUSZ D; FURHOLM T; GTARI M; REHAN M; SEN A; WALL LG; XU F

Porto

Congreso; 16 International meeting on Frankia and actinorhizal plants; 2010

Frankia are nitrogen-fixing actinobacteria that form root nodules with dicotyledonous plants in 8 families of angiosperms that are only distantly related to each other. Symbiotic interactions between Frankia and the host plant are not well understood. The nature of the chemical signals exchanged between the two partners of actinorhizal symbioses is still unknown due to the lack of genetic tools in Frankia and of specific molecular markers of the symbiotic interaction. While we have focused on resolving this situation by developing genetic tools, we have also pursued new genomic approaches toward studying these bacteria. Three Frankia genomes were sequenced completely and six more genomes are currently in the pipeline being sequenced. In the absence of genetic tools, the availability of these genome sequences also provides opportunities to use bioinformatic approaches and other new technologies. Analysis of the Frankia genomes has provided a myriad of information on these bacteria and several surprises including the extraordinary size discrepancy among the six Frankia genomes (5.4 Mb for strain CcI3 to 9.1 Mb for strain EAN1pec). The absence of obvious nodulation genes similar to those found in Rhizobia genomes suggests that the actinorhizal symbiosis uses novel signal compounds during the infection process. These genomes reveal potential new metabolic capabilities and tolerance mechanisms. An analysis of the draft genome sequence of Frankia EuI1c elucidated the absence of nitrogenase genes and may explain the Fix- nature of this infective strain. An overview of the comparative genomics of these Frankia genomes and other actinobacterial genomes including Mycobacteria will be presented.