THE HETEROCYST-SPECIFIC FDXH GENE-PRODUCT OF THE CYANOBACTERIUM ANABAENA SP PCC-7120 IS IMPORTANT BUT NOT ESSENTIAL FOR NITROGEN-FIXATION

To clarify the role of the heterocyst-specific [2Fe-2S] ferredoxin in cyanobacterial nitrogen fixation, mutational analysis of the Anabaena 7120 fdxH gene region was carried out, First, the DNA sequence of the wild-type 3509-bp EcoRI fragment downstream of the fdxH gene was deter mined. Genes homologous to ORF3 from the fdxH gene regions of A. varia bilis and Plectonema boryanum, the mop genes of Clostridium pasteurian um encoding molybdo-pterin binding proteins, and ORF3 from the A. vari abilis hydrogenase gene cluster were identified within the sequenced r egion. For mutational analysis the Anabaena 7120 mutant strains LAK4, BMB92, and KSH10 were constructed. In LAK4 the fdxH coding region is d isrupted by an interposon, whereas BMB92 is deleted for a 2799-bp NheI fragment encompassing fdxH, ORF3, mop, ORF4, and ORF5. Mutant strain KSH10 is a derivative of BMB92, complemented for fdxH but not for the other genes located further downstream Analysis of the Nif phenotype o f these mutant strains showed that FdxH is necessary for maximum nitro genase activity and optimal growth under nitrogen-fixing conditions, b ut not absolutely essential for diazotrophic growth, The role of alter native electron donors for nitrogenase, which might substitute for Fdx H. is discussed. Iron concentrations (1 mu M Fe) sufficient to induce synthesis of the vegetative cell flavodoxin did not stimulate diazotro phic growth of the fdxH mutant strains. suggesting that FdxH was not r eplaced by a NifJ-flavodoxin system. Comparison of LAK4 and BMB92 indi cated that one of the genes located downstream of fdxH might also play a (minor) role in nitrogen fixation.