Evidence against a common use of the diaphorase subunits by the bidirectional hydrogenase and by the respiratory complex I in cyanobacteria

The diaphorase subunits Hox(E)FU of the cyanobacterial bidirectional hydrogenase complex have been suggested to serve also as the three missing proteins of the cyanobacterial respiratory complex I. These subunits, encoded by nuoEFG in Escherichia coli, contain the NAD(+) and FMN binding sites. Previous physiological and molecular experiments demonstrated that neither the bidirectional hydrogenase activity nor hoxYH, encoding the hydrogenase dimer of the bidirectional enzyme, occur in the heterocystous cyanobacterium Nostoc PCC 73102. The present study demonstrates, by heterologous Southern blot hybridizations, that the genes hoxFU, encoding diaphorase subunits of the bidirectional enzyme, are both not present in Nostoc PCC 73102, whilst the genes hoxFU were detectable in all other heterocystous and unicellular cyanobacteria examined which possess the bidirectional hydrogenase. However, Nostoc PCC 73102 respires with rates comparable to those of other cyanobacteria and sequences similar to the genes ndhJ, ndhD2, ndhA and ndhI, encoding subunits of the respiratory complex I of Synechocystis PCC 6803, are present within the genome of Nostoc PCC 73102. Previous studies, using the unicellular strains Anacystis nidulans and Synechocystis PCC 6803, demonstrated that mutants in the diaphorase genes hoxU or hoxF are unable to evolve H-2, whereas the respiration is not affected. Altogether, these data are strongly against the hypothesis of a common use of the hox(E)FU gene products by the bidirectional hydrogenase and by the respiratory complex I in cyanobacteria. (C) 1999 Published by Elsevier Science B.V. All rights reserved.