Characterization of nifB, nifS, and nifU genes in the cyanobacterium Anabaena variabilis: NifB is required for the vanadium-dependent nitrogenase

Abstract

Anabaena variabilis ATCC 29413 is a heterotrophic, nitrogen-fixing cyanobacterium containing both a Mo-dependent nitrogenase encoded by the nif genes and V-dependent nitrogenase encoded by the vnf genes. The nifB, nifS, and nifU genes of A. variabilis were cloned, mapped, and partially sequenced. The fdxN gene was between nifB and nifS. Growth and acetylene reduction assays using wild-type and mutant strains indicated that the nifB product (NifB) was required for nitrogen fixation not only by the enzyme encoded by the nif genes but also by the enzyme encoded by the vnf genes. Neither NifS nor NifU was essential for nitrogen fixation in A. variabilis.Anabaena spp. are filamentous cyanobacteria capable of fixing dinitrogen in specialized morphologically distinct cells called heterocysts (18,41). Under diazotrophic growth conditions, vegetative cells compose 90 to 95% of the cells in a filament and contain both photosystems I and II. The other 5 to 10% of cells are heterocysts that have only photosystem I and therefore evolve no oxygen (40). Additional envelope layers in heterocysts may protect nitrogenase from oxygen (38, 39).Many nif genes have been identified in Anabaena sp. strain PCC 7120: one large cluster contains nifB, fdxN, nifS, nifU, nifH, nifD, nifK, nifN, nifX, open reading frame 3, nifW, open reading frame 1, open reading frame 2, and fdxH (8,23,30,33,36,37,42). nifJ is not part of the cluster (4). The nifB-fdxNnifS-nifU operon is interrupted by a 55-kb insertion in fdxN, and the nifD gene has an 11-kb insertion, both of which are excised during heterocyst differentiation (19,20). The heterocyst ferredoxin gene of Anabaena sp. strain PCC 7120, fdxH, is downstream of the other characterized nif genes in the cluster (8). The nif genes of A. variabilis ATCC 29413 that are the homologs of the nif genes of Anabaena sp. strain PCC 7120 have been cloned and mapped (24), and the 11-kb excision element, but not the 55-kb excision element, has been found in that nif cluster (9).Besides the nif genes that encode nitrogenase 1, which requires a Mo cofactor, Anabaena variabilis also contains vnf genes that encode nitrogenase 2, which requires a V cofactor (53). Nitrogenases that do not require Mo were first described in Azotobacter vinelandii, which has nitrogenase 1, nitrogenase 2, and nitrogenase 3. Nitrogenase 3, which requires only Fe in the cofactor, is encoded by the anf genes (6, 10, 27). The vnf and anf systems of A. vinelandii do not have homologs of all the genes of the nif system. Known vnf and anf genes include vnfDGK, homologous to nifDK; vnfEN, homologous to nifEN; vnfA, homologous to nifA; and anfHDGK, homologous to nif HDK (6). The nifB, nifS, nifU, nifM, and nifV genes are required for the activity of all three nitrogenases of A. vinelandii (28,29). The vnfEN genes may also function in the anf system (6). The vnf-and anf-encoded nitrogenases are repressed by Mo and have the ability to reduce acetylene not only to ethylene but also to ethane, which is one criterion for the presence of these altern...

GlbN (cyanoglobin) is a peripheral membrane protein that is restricted to certain Nostoc spp

“…Southern and Western analyses failed to identify any glbN or GlbN homologs in A. variabilis ATCC 29413. In addition, no glbN sequence was found in the nif1, nif2, fdxH1, or fdxH2 gene clusters of this strain (32,50,53).…”

High-Affinity Vanadate Transport System in the CyanobacteriumAnabaena variabilisATCC 29413

“…The 5-kb XhoI fragment of pRL1075 (6), which is required for conjugation of plasmids to A. variabilis, was inserted into the SalI site of pBP249 to create pBP250. Replacement of the wild-type vupB gene in the chromosome of strain FD by the mutant vupB allele in pBP250 was accomplished with conjugative nonreplicative plasmids as described previously (30). The mutant was segregated as described previously (45,46,51) and tested by PCR to verify that no wild-type copies of the gene remained.…”

Characterization of a nitrogen-fixation (nif) gene cluster from Anabaena azollae 1a shows that closely related cyanobacteria have highly variable but structured intergenic regions

“…Finally, the nucleotide sequence of the A . apollae la nifgenes is more than 99 % identical with that of Anabaena 2941 3 (Lyons & Thiel, 1995).…”

“…nifgenes have also been characterized from Anabaena sp. ATCC 2941 3 (Anabaena 29413) (Hirschberg et al, 1985;Thiel, 1993;Lyons & Thiel, 1995), Anabaena sp. L31 (Anabaena L31) (Murphy et al, 1993), Nostoc sp.…”