Frankia is a facultative symbiont actinomycete living with the dicotyledonous
plant worldwide. Two hydrogenase functions- uptake hydrogenase and hydrogenevolving
hydrogenase- have been so far reported in Frankia species, while the
presence of [Fe] hydrogenase function was surmised. Investigation of three Frankia
strains, ACN14a, CcI3 and R43 disclosed non-existence of the [Fe] hydrogenase
function, and bidirectional hydrogenase function in two of the Frankia strains,
ACN14a and CcI3. The blast of the three available Frankia genomes with the
conserved region of [Fe] hydrogenase, subunits of cyanobacterial bidirectional
hydrogenase, hyd and hyn genes resulted in no relevant information, but uptake
hydrogenase. Regarding the gene expression studies, it is supposed that [Fe]
hydrogenase is unlikely to presence in the Frankia strains. On the other hand,
bidirectional hydrogenase function is unlikely to occur in the strains ACN14a and
CcI3 since hydrogen evolution was not detected in these strains. The hydrogen
production in the strain R43 was found both under aerobic and anaerobic condition
regardless of nickel availability. Application of hox-specific primers to all the strains
evaluated did not provide an informative dataset. It is possible that hydrogen
evolution in the strain R43 was retrieved from uptake hydrogenase function acting in
reverse direction. In addition, it was shown that uptake hydrogenase was
consecutively expressed. However, knowledge about hydrogenase in Frankia is now
like the tip of the iceberg. It is not possible to show the presence of bidirectional
hydrogenase gene in the strain R43 until its genome is sequenced
