Characterization of genes for an alternative nitrogenase in the cyanobacterium Anabaena variabilis

Abstract

Anabaena variabilis ATCC 29413 is a heterotrophic, nitrogen-fixing cyanobacterium that has been reported to fix nitrogen and reduce acetylene to ethane in the absence of molybdenum. DNA from this strain hybridized well at low stringency to the nitrogenase 2 (vnfDGK) genes of Azotobacter vinelandii. The hybridizing region was cloned from a lambda EMBL3 genomic library of A. variabilis, mapped, and sequenced. The deduced amino acid sequences of the vnfD and vnfK genes of A. variabilis showed only about 56% similarity to the nifDK genes of Anabaena sp. strain PCC 7120 but were 76 to 86% similar to the anfDK or vnfDK genes of A. vinelandii. The organization of the vnf gene cluster in A. variabilis was similar to that of A. vinelandii. However, in A. variabilis, the vnfG gene was fused to vnfD; hence, this gene is designated vnfDG. A vnfH gene was not contiguous with the vnfDG gene and has not yet been identified. A mutant strain, in which a neomycin resistance cassette was inserted into the vnf cluster, grew well in a medium lacking a source of fixed nitrogen in the presence of molybdenum but grew poorly when vanadium replaced molybdenum. In contrast, the parent strain grew equally well in media containing either molybdenum or vanadium. The vnf genes were transcribed in the absence of molybdenum, with or without vanadium. The vnf gene cluster did not hybridize to chromosomal DNA from Anabaena sp. strain PCC 7120 or from the heterotrophic strains, Nostoc sp. strain Mac and Nostoc sp. strain ATCC 29150. A hybridizing ClaI fragment very similar in size to the A. variabilis ClaI fragment was present in DNA isolated from several independent, cultured isolates of Anabaena sp. from the Azolla symbiosis.

Lichen-symbiotic cyanobacteria associated withPeltigerahave an alternative vanadium-dependent nitrogen fixation system

“…Studies of the cyanobacterial genus Anabaena (a member of the same family as Nostoc, Nostocaceae) revealed that certain strains, all of which appear to be either obligate or facultative symbionts with plants, have an additional nitrogenase complex that is encoded by the vnf gene cluster (Thiel, 1993;Zehr et al, 2003;Raymond et al, 2004;Boison et al, 2006). The presence of the latter has also been inferred in a single strain of the cyanobacterial genus Nostoc (PCC 7422, isolated from plant tissue of Cycas sp.…”

Tansley Review No. 107. Heterocyst and akinete differentiation in cyanobacteria

“…However, the nif2 cluster, which lacks both the 11-kb and 55-kb elements, is expressed in vegetative cells, but only under anaerobic conditions, and in heterocysts (Thiel et al, , 1997. The vnf cluster codes for an alternative nitrogenase system containing a vanadium cofactor, instead of the usual molybdenum cofactor, and is expressed in the absence of molybdenum, but only in heterocysts (Thiel, 1993(Thiel, , 1996.…”

Bacterial Life and Dinitrogen Fixation at a Gypsum Rock

“…With regard to cyanobacteria, the occurrence of a vanadium nitrogenase has been inferred from both physiological and gene characterization studies of the heterocystous cyanobacterium Anabaena variabilis (15,36). Circumstantial evidence suggests that A. variabilis also possesses an iron nitrogenase (16), but this has not yet been corroborated by molecular data.…”

“…A vanadium nitrogenase has also been detected only in Anabaena isolates from the water fern Azolla and in Nostoc sp. strain strain PCC6720 (Anabaenopsis circularis) (36). In addition, cyanobacteria may possess two sets of genetically different Mo-containing nitrogenases, a unique feature which has been clearly shown for A. variabilis (32,37).…”