Nitrogen Fixation by Azotobacter vinelandii Strains Having Deletions in Structural Genes for Nitrogenase

Phenotypic reversal of Nif(-) mutant strains to Nif(+) under molybdenum-deficient conditions has been cited as evidence that Azotobacter vinelandii possesses two nitrogen fixation systems: the conventional molybdenum-enzyme system and an alternative nitrogen-fixation system. Since explanations other than the existence of an alternative system were possible, deletion strains of A. vinelandii lacking the structural genes for conventional nitrogenase (nifHDK) were constructed. These strains were found to grow in molybdenum-deficient nitrogen-free media, reduce acetylene (at low rates), and incorporate molecular nitrogen labeled with nitrogen-15. Thus it can be concluded that the phenotypic reversal phenomenon cannot be due to altered phenotypic expression of nif mutations under molybdenum-deficient conditions, but is due to the existence of an alternative nitrogen-fixation system in A. vinelandii as originally proposed.

Characterization of the iron superoxide dismutase gene of Azotobacter vinelandii: sodB may be essential for viability

Azotobacter vinelandii contains two superoxide dismutases (SODs), a cytoplasmic iron-containing enzyme (FeSOD), and a periplasmic copper/zinc-containing enzyme (CuZnSOD). In this study, the FeSOD was found to be constitutive, while the activity of CuZnSOD increased as the culture entered the stationary phase. Total SOD (units/mg protein) in stationary phase cells grown under nitrogen-fixing conditions was not significantly different from those grown under non-nitrogen-fixing conditions. The gene encoding FeSOD (sodB) was isolated from an A. vinelandii cosmid library. A 1-kb fragment containing the coding region and 400 base pairs of upstream sequence was cloned and sequenced. The nucleotide sequence and the deduced amino acid sequence had a high degree of homology with other bacterial FeSODs, particularly with P. aeruginosa. Attempts to construct a sodB mutant by recombination of a sodB::kan insertion mutation into the multicopy chromosome of A. vinelandii were unsuccessful even in the presence of SOD mimics or nutritional supplements. These results suggest that FeSOD may be essential for the growth and survival of A. vinelandii, and that the periplasmic CuZnSOD cannot replace the function of FeSOD.

Isolation of nitrogen-fixing bacteria containing molybdenum-independent nitrogenases from natural environments

Seven diazotrophs that grow well under Mo-deficient, N(2)-fixing conditions were isolated from a variety of environments. These isolates fall in the gamma subdivision of the class Proteobacteria and have genes that encode the Mo nitrogenase (nitrogenase 1) and the V nitrogenase (nitrogenase 2). Four of the isolates also harbor genes that encode the iron-only nitrogenase (nitrogenase 3).

Identification of genes unique to Mo-independent nitrogenase systems in diverse diazotrophs

A number of nitrogen-fixing bacteria were screened using PCR for genes (vnfG and anfG) unique to the V-containing nitrogenase (vnf) and the Fe-only nitrogenase (anf) systems. Products with sequences similar to that of vnfG were obtained from Azotobacter paspali and Azotobacter salinestris genomic DNAs, and products with sequences similar to that of anfG were obtained from Azomonas macrocytogenes, Rhodospirillum rubrum, and Azotobacter paspali DNAs. Phylogenetic analysis of the deduced amino acid sequences of anfG and vnfG genes shows that each gene product forms a distinct cluster. Furthermore, amplification of an internal 839-bp region in anfD and vnfD yielded a product similar to anfD from Heliobacterium gestii and a product similar to vnfD from Azotobacter paspali and Azotobacter salinestris. Phylogenetic analysis of NifD, VnfD, and AnfD amino acid sequences indicates that AnfD and VnfD sequences are more closely related to each other than either is to NifD. The results of this study suggest that Azotobacter salinestris possesses the potential to express the vanadium (V)-containing nitrogenase (nitrogenase 2) and that R. rubrum, Azomonas macrocytogenes, and H. gestii possess the potential to express the Fe-only nitrogenase (nitrogenase 3). Like Azotobacter vinelandii, Azotobacter paspali appears to have the potential to express both the V-containing nitrogenase and the Fe-only nitrogenase.

Regulation of the transcriptional activators AnfA and VnfA by metals and ammonium in Azotobacter vinelandii

Transcription of the genes encoding molybdenum (Mo)-independent nitrogenases 2 and 3 of Azotobacter vinelandii requires the activators VnfA and AnfA, respectively. The effect of NH4+, Mo, or V (vanadium) was tested on the expression of vnfA-lacZ and anfA-lacZ transcriptional fusions. Mo repressed expression of both fusions whereas NH4+ and V repressed the anfA-lacZ fusion, but not the vnfA-lacZ fusion. Thus the repressive effect on transcription of the anfHDGKOR operon by NH4+, Mo, or V is mediated through their effect on transcription of anfA and the repressive effect of Mo on the vnfHFd and vnfDGK operons is mediated through Mo repression of vnfA transcription. Mo-dependent repression of anfA transcription is influenced but not entirely mediated by the Mo-responsive regulator ModE.

Phenotypic characterization of a tungsten-tolerant mutant of Azotobacter vinelandii

A tungsten-tolerant mutant strain (CA6) of Azotobacter vinelandii first described in 1980 (P. E. Bishop, D. M. L. Jarlenski, and D. R. Hetherington, Proc. Natl. Acad. Sci. USA 77:7342-7346, 1980) has been further characterized. Results from growth experiments suggest that both nitrogenases 1 and 3 are utilized when CA6 grows in N-free medium containing Na2MoO4. Strain CA6.1.71, which lacks both nitrogenases 2 and 3, grew as well as strain CA in N-free medium containing Na2MoO4 after an initial lag. This indicates that nitrogenase 1 is fully functional in strain CA6. nifH-lacZ and anfH-lacZ transcriptional fusions were expressed in CA6 in the presence of Na2MoO4. Thus, in contrast to wild-type strain CA, transcription of the anfHDGK gene cluster in strain CA6 is not repressed by Mo. Expression of the vnfD-lacZ fusion was the same in both strains CA and CA6. In agreement with the results obtained with lac fusions, subunits of both nitrogenases 1 and 3 were found in protein extracts of CA6 cells grown in N-free medium containing Na2MoO4. However, CA6 cells, cultured in the presence of Na2WO4, accumulated nitrogenase 3 proteins without detectable amounts of nitrogenase 1 proteins. This indicates that expression of Mo-independent nitrogenase 3 is the basis for the tungsten tolerance phenotype of strain CA6. A measure of Mo accumulation as a function of time showed that accumulation by strain CA6 was slower than that for strain CA. When Mo accumulation was studied as a function of Na2MoO4 concentration, the two strains accumulated similar amounts of Mo in the concentration range of 0 to 1 microM Na2MoO4 during a 2-h period. Within the range of 1 to 5 microM Na2MoO4, Mo accumulation by strain CA increased linearly with increasing concentration whereas no further increases were observed for strain CA6. These results are consistent with the possibility that the tungsten tolerance mutation carried by CA6 is in a Mo transport system.

Characteristics of orf1 and orf2 in the anfHDGK genomic region encoding nitrogenase 3 of Azotobacter vinelandii

In Azotobacter vinelandii, the anfHDGK operon encodes the subunits for the third nitrogenase complex. Two open reading frames (orf1 and orf2) located immediately downstream of anfK were shown to be required for diazotrophic growth under Mo- and V-deficient conditions. We have designated orf1 and orf2 anfO and anfR, respectively. Strains (CA115 and CA116) carrying in-frame deletions in anfO and anfR accumulate the subunits for nitrogenase 3 under Mo-deficient diazotrophic conditions. AnfO and AnfR are required for nitrogenase 3-dependent diazotrophic growth and 15N2 incorporation but not for acetylene reduction. AnfO contains a putative heme-binding domain that exhibits similarity to presumed heme-binding domains of P-450 cytochromes. Amino acid substitutions of Cys-158 show that this residue is required for fully functional AnfO as measured by diazotrophic growth under Mo- and V-deficient conditions. The nucleotide sequence of the region located immediately downstream of anfR has been determined. A putative rho-independent transcription termination site has been identified 250 bp from the 3' end of anfR. A third open reading frame (orf3), located downstream of anfR, does not appear to be required for diazotrophic growth under Mo- and V-deficient conditions.

The genes encoding the delta subunits of dinitrogenases 2 and 3 are required for mo-independent diazotrophic growth by Azotobacter vinelandii

vnfG and anfG encode the delta subunits of alternative nitrogenases 2 and 3 in Azotobacter vinelandii, respectively. As a first step towards elucidating the role of these subunits, diazotrophic growth and acetylene reduction studies were conducted on mutants containing alterations in the genes encoding these subunits. Mutants containing a stop codon (C36stop) or an in-frame deletion in anfG were unable to grow in N-free, Mo-deficient medium (Anf-). Mutants in which cysteine 36 of AnfG (a residue conserved between VnfG and AnfG) was changed to Ala or Ser were Anf+. Thus, this conserved cysteine is not essential for the function of AnfG in dinitrogenase 3. A mutant with a stop codon in vnfG (C17stop) grew after a lag of 25 h in N-free, Mo-deficient medium containing V2O5. However, a Nif- Anf- strain with this mutation was unable to grow under these conditions. This shows that the vnfG gene product is required for nitrogenase 2-dependent growth. Strains with mutations in vnfG and anfG reduced acetylene to different degrees. This indicates that the delta subunits are not required for acetylene reduction by nitrogenases 2 and 3.

Effect of amino acid substitutions in a potential metal-binding site of AnfA on expression from the anfH promoter in Azotobacter vinelandii

AnfA, an activator required for transcription of the structural genes encoding nitrogenase 3 (anfHDGK) in Azotobacter vinelandii, has a potential metal-binding site [(S19)H(C21)FTGE(C26)R] in its N terminus. Growth studies and expression of an anfH-lacZ fusion in mutants containing amino acid substitutions in this site indicate that Ser-19 is not required for AnfA activity whereas Cys-21 and Cys-26 are required. Residual expression of the anfH-lacZ fusion in AnfA- mutants was found to be due to activation by VnfA, the activator required for expression of genes encoding nitrogenase 2.

Mo-independent nitrogenase 3 is advantageous for diazotrophic growth of Azotobacter vinelandii on solid medium containing molybdenum

Competition experiments between wild-type Azotobacter vinelandii and a mutant lacking Mo-independent nitrogenase 3 indicate that nitrogenase 3 provides an advantage during diazotrophic growth on agar media containing 100 to 500 nM Na2MoO4 but not in liquid media under the same conditions. Expression of nitrogenase 3 in wild-type cells growing on agar surfaces was verified with an anfH-lacZ fusion and by detection of nitrogenase 3 subunits. These results show that nitrogenase 3 is important for diazotrophic growth on agar medium at molybdenum concentrations that are not limiting for Mo-dependent diazotrophic growth in liquid medium.

Characterization of transcripts expressed from nitrogenase-3 structural genes of Azotobacter vinelandii

Five major anfH-hybridizing mRNA species accumulated in Azobacter vinelandii cells derepressed for nitrogenase-3 (an alternative nitrogenase, which appears to lack Mo and V). Using anfH-, anfD-, anfG-, anfK-, and orflorf2-specific probes and mutant strains of A. vinelandii these mRNA species have been identified as encoding anfHDGKorflorf2 (6.0 kb), anfHDGK (4.3 kb), anfHD (2.6 kb), vnfHorfFd (1.3 kb), and vnfH and (or) anfH (1.0 kb). A 0.6-kb mRNA species, which hybridized only to the orflorf2-specific probe, and a 3.5-kb mRNA species, which hybridized to anfD or anfK, also accumulated under these conditions. Northern blot analysis and S1 nuclease mapping indicate that transcription of the anf structural gene cluster initiates at a unique nif consensus promoter situated 127 base pairs upstream from the anfH coding region. Observation of anfH-hybridizing mRNA species that accumulate in strains derepressed for nitrogen fixation demonstrates that transcription of the anfHDGKorflorf2 cluster is normally repressed by Mo, V, and NH4+, whereas transcription of the vnfHorfFd cluster does not require the presence of V and is repressed only by Mo, but not NH4+. Analysis of the accumulation of mRNAs in a tungsten-tolerant strain revealed that Mo and V repression of anf transcription must occur by different mechanisms.

Regulation of nitrogenase-2 in Azotobacter vinelandii by ammonium, molybdenum, and vanadium

Under diazotrophic conditions in the absence of molybdenum and in the presence of vanadium, Azotobacter vinelandii reduces N2 to NH4+ by using nitrogenase-2, a V-containing enzyme complex encoded by vnfH (the gene for dinitrogenase reductase-2), and vnfDGK (the genes for dinitrogenase-2 subunits). Accumulation of the vnfHorfFd and vnfDGK transcripts occurred under Mo-deficient conditions in the presence and absence of V; however, in the case of vnfDGK, the protein products only accumulated in the presence of V. This suggests that V is required for translation of the vnfDGK transcripts. In addition, expression of vnfH-lacZ and vnfD-lacZ transcriptional fusions was only partially repressed in the presence of NH4+. Transcripts hybridizing with vnfH (1.4 and 1.0 kb), vnfDG (3.4 and 1.8 kb), and vnfK (3.4 kb) were detected in RNA extracted from wild-type cells cultured with NH4+ in the presence or absence of V. However, nitrogenase-2 subunits were not detected in extracts of cells derepressed for nitrogenase-2 in the presence of NH4+. These results indicate that this nitrogen source acts at the posttranscriptional level as well as at the transcriptional level. vnf transcripts were not detected in the presence of Mo (with or without NH4+).

Nucleotide sequence and mutational analysis of the vnfENX region of Azotobacter vinelandii

The nucleotide sequence (3,600 bp) of a second copy of nifENX-like genes in Azotobacter vinelandii has been determined. These genes are located immediately downstream from vnfA and have been designated vnfENX. The vnfENX genes appear to be organized as a single transcriptional unit that is preceded by a potential RpoN-dependent promoter. While the nifEN genes are thought to be evolutionarily related to nifDK, the vnfEN genes appear to be more closely related to nifEN than to either nifDK, vnfDK, or anfDK. Mutant strains (CA47 and CA48) carrying insertions in vnfE and vnfN, respectively, are able to grow diazotrophically in molybdenum (Mo)-deficient medium containing vanadium (V) (Vnf+) and in medium lacking both Mo and V (Anf+). However, a double mutant (strain DJ42.48) which contains a nifEN deletion and an insertion in vnfE is unable to grow diazotrophically in Mo-sufficient medium or in Mo-deficient medium with or without V. This suggests that NifE and NifN substitute for VnfE and VnfN when the vnfEN genes are mutationally inactivated. AnfA is not required for the expression of a vnfN-lacZ transcriptional fusion, even though this fusion is expressed under Mo- and V-deficient diazotrophic growth conditions.

The gene encoding dinitrogenase reductase 2 is required for expression of the second alternative nitrogenase from Azotobacter vinelandii

Under diazotrophic conditions in the absence of molybdenum (Mo) and vanadium (V), Azotobacter vinelandii reduces N2 to NH4+ by using nitrogenase 3 (encoded by anfHDGK). However, dinitrogenase reductase 2 (encoded by vnfH) is also expressed under these conditions even though this protein is a component of the V-containing alternative nitrogenase. Mutant strains that lack dinitrogenase reductase 2 (VnfH-) grow slower than the wild-type strain in N-free, Mo-, and V-deficient medium. In this medium, these strains synthesize dinitrogenase reductase 1 (a component of the Mo-containing nitrogenase encoded by nifH), even though this component is not normally synthesized in the absence of Mo. Strains that lack both dinitrogenase reductases 1 and 2 (NifH-VnfH-) are unable to grow diazotrophically in Mo- and V-deficient medium. In this medium, NifH- VnfH- strains containing an anfH-lacZ transcriptional fusion exhibited less than 3% of the beta-galactosidase activity observed in the wild type with the same fusion. Beta-Galactosidase activity expressed by VnfH- mutants containing the anfH-lacZ fusion ranged between 57 and 78% of that expressed by the wild type containing the same fusion. Thus, expression of dinitrogenase reductase 2 seems to be required for transcription of the anfHDGK operon, although, in VnfH-mutants, dinitrogenase reductase 1 appears to serve this function. Active dinitrogenase reductase 1 or 2 is probably required for this function since a nifM deletion mutant containing the anfH-lacZ fusion was unable to synthesize beta-galactosidase above background levels. An anfA deletion strain containing the anfH-lacZ fusion exhibited beta-galactosidase activity at 16% of that of the wild type containing the same fusion. However, in the presence of NH4+, the beta-galactosidase activity expressed by this strain more than doubled. This indicates that AnfA is required not only for normal levels of anfHDGK transcription but also for NH4+ -and, to a lesser extent, Mo-mediated repression of this transcription.

Sensitivity of digoxigenin and biotin labelled probes for detection of human papillomavirus by in situ hybridisation

The sensitivity of digoxigenin and biotin labelled DNA probes for the detection of human papillomavirus (HPV) by dot blotting and in situ hybridisation was compared in tissues from cervical, laryngeal, and anogenital neoplasia. Probes were either labelled with digoxigenin by the random primer technique and detected with anti-digoxigenin antibody, or labelled with biotin by nick translation and detected with streptavidin, both methods having a common final visualisation procedure using alkaline phosphatase. Digoxigenin labelled probes proved two to 10-fold more sensitive by quantitative dot blotting and four-fold more sensitive in detecting HPV 16 DNA in a series of 31 anal carcinomas, compared with biotinylated probes. The digoxigenin method also produced less non-specific background staining of tissue sections than biotin labelled probes. It is concluded that digoxigenin DNA labelling and detection provides a simple, reliable, and efficient alternative to the use of biotin or radioactive isotopes for the detection of HPV DNA by in situ hybridisation. Digoxigenin labelled probes also offer the possibility of double labelling in situ hybridisation procedures when used with biotin labelled probes to provide simultaneous identification of different DNA sequences.

An immunohistological study of condylomata acuminata

Using an indirect immunoperoxidase method and a panel of monoclonal antibodies, anogenital warts removed from 86 individuals (28 heterosexual men, 47 homosexual men and 11 women) were studied. A constant feature was the presence of a stromal infiltrate of lymphocytes, in which CD4+ and CD8+ cells appeared in a ratio of about 5:1. Cells expressing interleukin-2 receptors were found in the stroma, amounting to 10% of the CD3+ cells. Leu 6+ epidermal dendritic cells (Langerhans' cells) were present in each wart; their density and morphology were similar to that in adjacent, apparently normal, skin. The mean number of Langerhans' cells that expressed HLA-DQ was lower than that of HLA-DR + cells. In 40 warts, HLA-DR was expressed on the surface of keratinocytes. Leu 7+ and Leu 11b+ cells were found in the epidermis of 17 and five warts, respectively. B-cells were noted in only five warts. These results are discussed in relation to persistence of wart virus infection.

Nucleotide sequences and mutational analysis of the structural genes for nitrogenase 2 of Azotobacter vinelandii

The nucleotide sequence (6,559 base pairs) of the genomic region containing the structural genes for nitrogenase 2 (V nitrogenase) from Azotobacter vinelandii was determined. The open reading frames present in this region are organized into two transcriptional units. One contains vnfH (encoding dinitrogenase reductase 2) and a ferredoxinlike open reading frame (Fd). The second one includes vnfD (encoding the alpha subunit of dinitrogenase 2), vnfG (encoding a product similar to the delta subunit of dinitrogenase 2 from A. chroococcum), and vnfK (encoding the beta subunit of dinitrogenase 2). The 5'-flanking regions of vnfH and vnfD contain sequences similar to ntrA-dependent promoters. This gene arrangement allows independent expression of vnfH-Fd and vnfDGK. Mutant strains (CA80 and CA11.80) carrying an insertion in vnfH are still able to synthesize the alpha and beta subunits of dinitrogenase 2 when grown in N-free, Mo-deficient, V-containing medium. A strain (RP1.11) carrying a deletion-plus-insertion mutation in the vnfDGK region produced only dinitrogenase reductase 2.

An immunohistological study of spontaneous regression of condylomata acuminata

An immunohistological study of four men whose perianal warts were undergoing spontaneous regression was undertaken, and the results compared with those obtained from non-regressing condylomata from six men. CD4+ and CD8+ cells were noted in the stroma of each wart, but there was no clear difference in the density of the infiltrate between regressing and non-regressing warts. Natural killer cells (CD16+ and CD57+) were only noted in the stroma and epidermis of regressing warts. Possible immunological mechanisms of regression of condylomata acuminata are discussed.

Immunological study of condylomata acuminata in men infected with the human immunodeficiency virus

As condylomata acuminata often persist in individuals infected with the human immunodeficiency virus (HIV), an immunohistological study of warts from infected men was undertaken to further knowledge about human papillomavirus persistence in this group. Using an indirect immunoperoxidase method and a panel of monoclonal antibodies, the phenotypes of cells were studied in cryostat sections of perianal or anal warts removed from 14 HIV-infected men (10 homosexual and 4 heterosexual) and from 16 non-infected men (10 homosexual and 6 heterosexual). Although the median numbers of CD1+, CD3+ and CD4+ cells per unit area were similar in each group of individuals, the number of CD8+ cells was significantly higher in HIV-infected homosexual men when compared with non-infected individuals and HIV-infected heterosexual men. The median CD4+ cell count in the peripheral blood was significantly higher in HIV-infected heterosexual men than in HIV-infected homosexual men (P less than 0.05). These findings may reflect differences in duration of HIV infection between the two groups. There was no significant difference in the proportion of cells expressing interleukin-2 receptors between HIV-infected and non-infected individuals. Natural killer (CD16+) cells were not identified in any of the condylomata.

Clinical course of anogenital warts in men infected with human immunodeficiency virus

Fifty four men with anogenital warts were studied; 22 had concurrent infection with the human immunodeficiency virus (HIV). The median duration of the warts before and after the start of treatment of seven HIV infected and 10 non-infected heterosexual men was similar. In homosexual men, however, the duration of the lesions in 15 HIV infected patients was greater before and after treatment than in 22 non-HIV infected men. As the median number of CD4+ cells in the peripheral blood was significantly lower in homosexual than heterosexual men infected with HIV, the difference in the course of anogenital warts in homosexual compared with heterosexual men may reflect different degrees of immunosuppression.

Two nifA-like genes required for expression of alternative nitrogenases by Azotobacter vinelandii

Two nifA-like genes, designated anfA and vnfA, have been identified in Azotobacter vinelandii. The anfA gene is located upstream from the nitrogenase-3 structural gene cluster (anfHDGK) and is preceded by a sequence that is potentially part of a ntrA-dependent promoter. The product of anfA appears to be required for expression of nitrogenase-3, since cells of the anfA deletion strain CA66 were unable to synthesize this nitrogenase when derepressed in N-free, Mo- and V-deficient medium. The vnfA gene was identified after determination of the nucleotide sequence of DNA flanking the Tn5 insertion in mutant strain CA46. Two open reading frames (ORF1 and ORF2) were found located upstream from the vnfA gene, and a nifE-like ORF, preceded by a possible ntrA-dependent promoter, was found downstream from this gene. It is not known whether vnfA is expressed only under N2-fixing conditions. However, potential ntrA-dependent promoters were found immediately upstream from vnfA (within the 3' end of ORF2) and immediately downstream from ORF1. The region spanning ORF1 and ORF2 contained an A + T-rich sequence that was also found immediately upstream from the potential ntrA-dependent promoter of anfA. The product of vnfA appears to be required for the synthesis of nitrogenase-2, since cells of strain CA46 synthesized only nitrogenase-1 and -3 but not nitrogenase-2 when grown in the presence of vanadium. The product of nifA, which is required for synthesis of nitrogenase-1, is not required for synthesis of either nitrogenase-2 or nitrogenase-3. However, growth data indicate that nifA is required for a factor (or factors) necessary for maximal diazotrophic growth under Mo- and V-deficient conditions.

Immunohistology of the skin rash associated with acute HIV infection

An immunohistological study of the rash associated with acute HIV infection in a homosexual man was undertaken. The salient feature was an infiltrate of mononuclear cells predominantly of the CD4+ phenotype around the superficial dermal vessels. HIV p24 antigen was detected in occasional cells, possibly Langerhans' cells, of the infiltrate. Their presentation of HIV antigen to local CD4+ cells may lead to a delayed type hypersensitivity reaction.

Nucleotide sequence and mutational analysis of the structural genes (anfHDGK) for the second alternative nitrogenase from Azotobacter vinelandii

The nucleotide sequence of a region of the Azotobacter vinelandii genome exhibiting sequence similarity to nifH has been determined. The order of open reading frames within this 6.1-kilobase-pair region was found to be anfH (alternative nitrogen fixation, nifH-like gene), anfD (nifD-like gene), anfG (potentially encoding a protein similar to the product of vnfG from Azotobacter chroococcum), anfK (nifK-like gene), followed by two additional open reading frames. The 5'-flanking region of anfH contains a nif promoter similar to that found in the A. vinelandii nifHDK gene cluster. The presumed products of anfH, anfD, and anfK are similar in predicted Mr and pI to the previously described subunits of nitrogenase 3. Deletion plus insertion mutations introduced into the anfHDGK region of wild-type strain A. vinelandii CA resulted in mutant strains that were unable to grow in Mo-deficient, N-free medium but grew in the presence of 1 microM Na2MoO4 or V2O5. Introduction of the same mutations into the nifHDK deletion strain CA11 resulted in strains that grew under diazotrophic conditions only in the presence of vanadium. The lack of nitrogenase 3 subunits in these mutant strains was demonstrated through two-dimensional gel analysis of protein extracts from cells derepressed for nitrogenase under Mo and V deficiency. These results indicate that anfH, anfD, and anfK encode structural proteins for nitrogenase 3.

Nucleotide sequence and genetic analysis of the nifB-nifQ region from Azotobacter vinelandii

A 3.8-kilobase-pair EcoRI fragment which corrects the mutations carried by the NifB- Azotobacter vinelandii strains CA30 and UW45 was cloned, and its nucleotide sequence was determined. Four complete open reading frames (ORFs) and two partial ORFs were found. The translation product of the first partial ORF is the carboxy-terminal end of a protein homologous to the nifA gene product from Klebsiella pneumoniae. A 285-base-pair sequence containing a potential nif promoter and nif regulatory sites separates this nifA gene from the first complete ORF which encodes a protein homologous to nifB gene products from K. pneumoniae and Rhizobium species. The Tn5 insertion in strain CA30 and the nif-45 mutation of strain UW45 are located within this nifB gene. The ORF downstream from nifB predicts an amino acid sequence with a cysteine residue pattern that is characteristic of ferredoxins. No similarities were found between the translation product of the third complete ORF and those of nif genes from other organisms. At the carboxy-terminal end of the predicted translation product of the fourth complete ORF, 30 of 60 amino acid residues were identical with the sequence of the nifQ gene product from K. pneumoniae. The partial ORF located at the end of the fragment encodes the N-terminal part of a potential protein with an unknown function. Northern (RNA) blot analysis indicated that transcripts from the region containing the four complete ORFs were NH4+ repressible and that the transcription products were identical in cells derepressed under conditions of Mo sufficiency or Mo deficiency or in the presence of vanadium. In contrast to the NifB- strain CA30, which is Nif- under all conditions, mutants that carry mutations affecting the C-terminal end of nifB or genes located immediately downstream from nifB, grew under all N2-fixing conditions. However, in the presence of Mo, most of the strains required 1,000 times the amount of molybdate that is sufficient for maximal growth of the wild-type strain CA under N2-fixing conditions. Growth data from strain CA37, which carries a Kanr insertion in nifQ, indicate that nifQ in A. vinelandii is not required for N2 fixation in the presence of V2O5 or under Mo-deficient conditions. Growth studies and acetylene reduction assays performed on two nifEN deletion strains showed that nifE and nifN are required for N2 fixation under Mo sufficiency, as previously observed (K. E. Brigle, M. C. Weiss, W. E. Newton, and D. R. Dean, J. Bacteriol. 169:1547-1553, 1987), but not under conditions of Mo deficiency or in the presence of 50 nM V2O5.

Bacterial alternative nitrogen fixation systems

The introduction briefly reviews some of the salient features of the well-characterized conventional molybdo-enzyme system for N2 fixation. This is followed by a brief account of the discovery of an alternative N2 fixation system that does not require molybdenum in the N2-fixing bacterum Azotobacter vinelandii. The next section cites observations from the early literature on N2 fixation suggesting may not always require molybdenum. Next, recent evidence for an alternative N2 fixation system in A. vinelandii is discussed. A brief description of our discovery of an alternative nitrogenase which is not a molybdenum or vanadium enzyme is presented, followed by a summary of recent papers describing an alternative vanadium-containing nitrogenase. Available information on the genetics and regulation of alternative N2 fixation systems is discussed. Finally, the possible/probable presence of alternative N2 fixation systems in bacteria other than Azotobacter species is covered.

Purification of a second alternative nitrogenase from a nifHDK deletion strain of Azotobacter vinelandii

A second alternative nitrogenase complex (nitrogenase 3) was purified from a nifHDK deletion strain of Azotobacter vinelandii. The active complex is made up of two components, dinitrogenase 3 and dinitrogenase reductase 3. Dinitrogenase 3 contains two protein subunits (alpha, Mr 58,000, and beta, Mr 50,000) which assemble into at least two active configurations: alpha 2 beta 2 (dinitrogenase 3s) and alpha 1 beta 2 (dinitrogenase 3F). Dinitrogenase 3s contains 24 Fe and 18 acid-labile S2-ions per Mr 216,000, and dinitrogenase 3F contains 11 Fe and 9 acid-labile S2-ions per Mr 158,000. Dinitrogenase reductase 3 is composed of two protein subunits of identical Mr (32,500) and contains four Fe and four acid-labile S2- ions per Mr 65,000. On two-dimensional gels, the protein subunits of the nitrogenase 3 complex comigrated with the four Mo-, V-, and NH4+-repressible proteins originally designated as N2ase B: the nitrogenase hypothesized to exist in the alternative N2 fixation system first described in 1980 (P.E. Bishop, D. M. L. Jarlenski, and D. R. Hetherington, Proc. Natl. Acad. Sci. USA 77:7342-7346, 1980). Neutron activation analysis indicated that the nitrogenase 3 complex lacked significant amounts of Mo, V, Cr, Re, and W. Some Zn, however, was found in the dinitrogenase 3S and dinitrogenase 3F preparations. The pattern of substrate reduction efficiency was H+ greater than N2 greater than C2H2. The maximum specific activity found for N2 reduction was 38 nmol of NH3 per min per mg of protein (dinitrogenase 3S). Nitrogenase 3 was found to be extremely sensitive to O2, and activities could not be reproducibly maintained during freezing and thawing.

Immunological study of the rectal mucosa of men with and without human immunodeficiency virus infection

Biopsies of rectal mucosa were taken from 81 men and stained using cytochemical methods for B and T lymphocytes, T cell subsets, immunoglobulin containing plasma cells and mucosal mast cells. The patients studied included human immunodeficiency virus (HIV) infected and non-infected heterosexual and homosexual men, and homosexual men with rectal gonorrhoea. There were increased numbers of T lymphocytes in the lamina propria of the rectum in HIV infected individuals regardless of whether the infection had been acquired through anal intercourse or intravenous drug use. This increase resulted from a marked increase in the numbers of CD8+ suppressor T cells, there also being a reduction in the numbers of CD4+ helper T cells. In non-HIV infected men with rectal gonorrhoea there were increased numbers of CD8+ T cells but no significant difference in numbers of CD4+ cells. No difference was seen in numbers of CD4+ cells. No difference was seen in numbers of immunoglobulin containing plasma cells or mucosal mast cells between HIV infected and non-infected men.

A histological and immunocytochemical study of lymphoid tissue in rectal biopsies from homosexual men

A histological study of the lymphoid tissue of the rectums of 323 homosexual and 30 heterosexual men (including five intravenous drug users seropositive for human immunodeficiency virus, HIV) was undertaken. Marked lymphoid hyperplasia (germinal centre greater than 680 microns in diameter) was found in 13 men, nine of whom had a sexually transmitted infection of the rectum. Immunocytochemical studies showed that the germinal centres contained B-cells and follicular dendritic cells, surrounded by CD4+ and CD8+ T-cells in a ratio of between 5:1 and 10:1. In HIV-infected men there were decreased numbers of CD4+ cells and increased numbers of CD8+ cells, resulting in a ratio of between 0.8:1 and 2:1. In biopsies from four of seven HIV-infected men the follicular dendritic cell processes appeared shorter and fragmented. The pathogenesis of HIV infection is discussed in the light of these findings.

Tn5-induced mutants of Azotobacter vinelandii affected in nitrogen fixation under Mo-deficient and Mo-sufficient conditions

Mutants of Azotobacter vinelandii affected in N2 fixation in the presence of 1 microM Na2MoO4 (conventional system), 50 nM V2O5, or under Mo deficiency (alternative system) have been isolated after Tn5 mutagenesis with the suicide plasmid pSUP1011. These mutants can be grouped into at least four broad phenotypic classes. Mutants in the first class are Nif- under Mo sufficiency but Nif+ under Mo deficiency or in the presence of V2O5. A nifk mutant and a mutant apparently affected in regulation of the conventional system belong to this class. Mutants in the second class are Nif- under all conditions. An FeMo-cofactor-negative mutant (NifB-) belongs to this class, implying an involvement of nifB in both the conventional and the alternative N2 fixation systems. The third mutant class consists of mutants incapable of N2-dependent growth under Mo deficiency. Most of the mutants in this class are also affected in N2 fixation in the presence of 1 microM Na2MoO4, with acetylene reduction rates ranging from 28 to 51% of the rates of the wild type. Strains constructed by genetic transfer of the Kanr marker of mutants from this class into nifHDK or nifK deletion mutants showed N2-dependent growth only in the presence of V2O5, suggesting that growth in the presence of V2O5 and growth under Mo deficiency are independent phenomena. The only mutant in the fourth class shows wild-type nitrogenase activity under Mo sufficiency, but only 10% of the acetylene reduction activity of the wild type in the presence of 50 nM V2O5. The acetylene reduction rates of whole cells of this mutant are identical in Mo-deficient medium and in medium containing V2O5. The conventional nitrogenase subunits are expressed in this mutant even under Mo deficiency or in the presence of V2O5; however, the NH4+- and Mo-repressible proteins normally seen under these conditions could not be detected on two-dimensional gels. The Tn5 insertion carried by this mutant makes N2 fixation dependent solely on the conventional system and consequently abolishes the vanadium effect.

Nitrogen fixation in molybdenum-deficient continuous culture by a strain of Azotobacter vinelandii carrying a deletion of the structural genes for nitrogenase (nifHDK)

Steady-state chemostat cultures of Azotobacter vinelandii strain CA11, carrying a deletion of genes encoding the structural polypeptides of nitrogenase nifHDK, were established in a simple defined medium chemically purified to minimize contamination by Mo. The medium contained no utilizable N source. Growth was dependent on N2 (1.1 X 10(8) viable cells X ml-1 at D = 0.176 h-1), and was inhibited by Mo (20 nM). DNA hybridization showed the deletion to be stable during prolonged (55 days) growth in the chemostat (132 doublings). Since batch cultures, using unsupplemented 'spent' chemostat medium, showed good growth (1.9 X 10(8) cells X ml-1), no requirement for subnanomolar concentrations of Mo was found. The biomass yield, as the dilution rate (D) was varied, showed that the N content of the culture, protein and dry wt. increased as D was decreased, indicating that neither N2 nor O2 were limiting growth. The limiting nutrient was not identified. Substantial amounts of H2 were evolved by the chemostat cultures, probably as the result of inhibition of O2-dependent hydrogenase activity by nitrilotriacetic acid present in the medium. Over a range of D values approx. 50% of the electron flux through the alternative system was allocated to H+ reduction. C2H2 was a poor substrate, being reduced at 0.14-0.1 times the rate of N2 fixation, calculated from the N content of the cells. SO4(2-)-limited steady-state continuous cultures of strain UW136 (wild-type for nifHDK) had a 2-fold greater biomass in the presence of MoO4(2-) (1 microM). The significance of this finding for 'Mo-limited' continuous cultures [Eady & Robson (1984) Biochem. J. 224, 853-862] is discussed.

Transcriptional regulation of nitrogen fixation by molybdenum in Azotobacter vinelandii

Multiple genomic regions homologous to nifH were found in the diazotroph Azotobacter vinelandii. The nifHDK gene cluster, located on a 12.8-kilobase (kb) XhoI fragment and two additional XhoI fragments (7.4 and 8.4 kb) hybridized to a nifH-specific DNA template but the 7.4- and 8.4-kb fragments did not hybridize to nifD- or nifK-specific DNA probes. In vivo transcription of the nifHDK gene cluster was ammonia-repressible and required the presence of at least 50 nM molybdenum in the derepression medium. Three mRNA species were transcribed from the nifHDK gene cluster, a 4.2-kb transcript homologous to nifH-, nifD-, and nifK-specific DNA templates, a 2.6-kb transcript homologous to nifH- and nifD-specific DNA templates, and a 1.2-kb transcript homologous only to the nifH-specific DNA template. In strain CA11, a nifHDK deletion mutant, the nifHDK-specific transcripts were not produced and the strain was unable to grow in N-free medium in the presence of Na2MoO4 at concentrations of 50 nM or higher. However, at concentrations of 25 nM Mo or less, growth occurred in N-free medium. Under these conditions two nifH-homologous (but not nifD- or nifK-homologous) transcripts were observed (1.2 and 1.8 kb). Presumably these were transcribed from the additional nifH-homologous sequences present in the genome. These results are consistent with the existence of two N2 fixation systems in A. vinelandii which are regulated by molybdenum at the level of transcription.

Molecular cloning of nif DNA from Azotobacter vinelandii

Two clones which contained nif DNA were isolated from a clone bank of total EcoRI-digested Azotobacter vinelandii DNA. The clones carrying the recombinant plasmids were identified by use of the 32P-labeled 6.2-kilobase (kb) nif insert from pSA30 (which contains the Klebsiella pneumoniae nifK, nifD, and nifH genes) as a hybridization probe. Hybridization analysis with fragments derived from the nif insert of pSA30 showed that the 2.6-kb insert from one of the plasmids (pLB1) contains nifK whereas the 1.4-kb insert from the other plasmid (pLB3) contains nifD. Marker rescue tests using genetic transformation indicated that the 2.6-kb A. vinelandii nif fragment contains the wild-type alleles for the nif-6 and nif-38 mutations carried by Nif- strains UW6 and UW38. The 1.4-kb insert contains the wild-type allele for the nif-10 mutation carried by Nif- strain UW10.

Expression of an alternative nitrogen fixation system in Azotobacter vinelandii

Nitrogenase activities were determined from maximum acetylene reduction rates for mutant strains of Azotobacter vinelandii which are unable to fix N2 in the presence of molybdenum (Nif-) but undergo phenotypic reversal to Nif+ under conditions of Mo deficiency. The system responsible for N2 fixation under these conditions is thought to be an alternative N2 fixation system (Bishop et al., Proc. Natl. Acad. Sci. U.S.A. 77:7342-7346, 1980). Phenotypic reversal of Nif- strains to Nif+ strains was also observed in N-free medium without Mo but with either V or Re. Two protein patterns were found on two-dimensional gels of proteins from the extracts of wild-type cells cultured in N-free medium without Mo and with or without V or Re. The expression of each protein pattern in the wild-type strain of A. vinelandii seemed to depend upon the physiological state of the N2-fixing culture. Electron paramagnetic resonance experiments were conducted on whole cells of A. vinelandii grown under conditions of Mo deprivation in the absence of fixed N. No g = 3.65 signal (an electron paramagnetic resonance signal characteristic of the Mo-containing component of nitrogenase) was detectable in these cells, regardless of whether V or Re was present during growth of these cells, These results are discussed from the perspective that the well-known effect of V on N2 fixation by A. vinelandii may involve an alternative N2 fixation system.

Evidence for an alternative nitrogen fixation system in Azotobacter vinelandii

Two Azotobacter vinelandii strains capable of growing on N2(Nif+) were isolated from two different mutant strains that lacked dinitrogenase activity (Nif-). Extracts of N2-grown cells of the two Nif+ strains lacked significant amounts of the "conventional" dinitrogenase protein subunits, as determined by two-dimensional gel electrophoresis. Instead, the extracts contained at least four new proteins that appeared to be ammonia-repressible (i.e., they were not detected in extracts of ammonia-grown cells). Based on the results of genetic backcrosses, the two Nif+ strains were shown to be pseudorevertants. Both Nif+ pseudorevertant strains were able to grow in N-free media lacking molybdenum but containing tungsten (conditions that prevented growth of the wild-type strain). The four new proteins were observed in extracts of N2-fixing cells of the Nif+ pseudorevertants regardless of whether the cells were grown in the presence of molybdenum-starved wild-type A. vinelandii cells grown under N2-fixing conditions. Under conditions of molybdenum deprivation, Nif- mutant strains of several different phenotypic classes underwent phenotypic reversal to Nif+, as shown by their ability to incorporate 15N2 and to grow in N-free media. These results provide evidence that A. vinelandii possesses an alternative N2-fixation system that is expressed during conditions of molybdenum deficiency.

Transfer from Rhizobium japonicum to Azotobacter vinelandii of genes required for nodulation

A mutant strain of Azotobacter vinelandii that is unable to fix N2 (Nif-) was transformed to Nif+ with DNA from Rhizobium japonicum. Of 50 Nif+ transformants tested, 3 contained the O antigen-related polysaccharide that is present on the cell surface of a nodulating R. japonicum strain, but is absent from a non-nodulating mutant strain.

Intergeneric transfer of genes involved in the Rhizobium-legume symbiosis

Genes that seem to be involved in the initial steps of infection of a legume by Rhizobium have been transferred, by transformation, to mutant strains of Azotobacter vinelandii that are unable to fix nitrogen. These genes code for a surface antigen that binds specifically to a protein from the host plant.

Genetic analysis of Azotobacter vinelandii mutant strains unable to fix nitrogen

Transformation was used to perform ratio test crosses with mutant strains of Azotobacter vinelandii unable to fix N2. Mutations that simultaneously eliminated both components of nitrogenase (nif-1 and nif-2) were tightly linked. The nif-45 mutation that resulted in the absence of an active molybdenum cofactor was closer to nif-1 and nif-2 than to any of the other nif mutations. Strains that lacked component I carried mutations that were closely linked to each other. Mutations that probably were located in the structural genes for components I and II appeared to be relatively close to each other on the A. vinelandii genome.

Technique for isolating phage for Azotobacter vinelandii

An enrichment technique was developed whereby azotophage could readily be isolated after inoculation of soil sites with Azotobacter vinelandii.

Relation between Glutamine Synthetase and Nitrogenase Activities in the Symbiotic Association between Rhizobium japonicum and Glycine max

The activity and extent of adenylylation of glutamine synthetase was examined in both free-living and bacteroid forms of Rhizobium japonicum in the presence of excess ammonia. Ammonia caused an apparent repression of glutamine synthetase in free-living R. japonicum and adenylylation of the enzyme was also increased. In contrast, neither the activity nor the extent of adenylylation of the bacteroid enzyme was consistently affected by ammonium treatment of bacteroid suspensions. Similar results were obtained after ammonium treatment of soybean plants even though nitrogenase activity was reduced markedly. We have been unable to demonstrate ammonium repression of nitrogenase activity in R. japonicum-Glycine max symbiotic association that is mediated through bacteroid glutamine synthetase. This result is in contrast to the situation in nitrogen-fixing strains of Klebsiella where a role of glutamine synthetase in the regulation of nitrogenase has been reported.

Bacillus subtilis GSY 1057 assay for aflatoxin B activation by rainbow trout (Salmo gairdneri)

A rapid and sensitive microbial assay was developed to detect lethal products of aflatoxin B metabolism by rainbow trout (Salmon gairdneri) Mt. Shasta strain. Bacillus subtilis GSY 1057 (hisA1, uvr-1, metB4), a DNA repair deficient strain, was incubated for 20 min in the 20,000 times g supernate from trout liver homogenates which had been preincubated for 10 min with various levels of aflatoxin B. Serial dilutions of the incubation mixture were plated in triplicate on tryptose blood agar base plates and colonies were counted after 12 hr at 37 degrees C. One mumole aflatoxin B in 3.2 ml incubation mixture reduced viability 60%.

Immunological evidence for the capability of free-living Rhizobium japonicum to synthesize a portion of a nitrogenase component

Immunodiffusion tests conducted under aerobic conditions demonstrated that cross-reactive material to antiserum prepared against the Mo-Fe protein component of nitrogenase from soybean nodule bacteroids was detectable in extracts of free-living Rhizobium japonicum cells cultured in a standard medium under: aerobic conditions; aerobic conditions with nitrate; aerobic conditions with ammonia; anaerobic conditions with nitrate; and anaerobic conditions with nitrate and ammonia. The most intense precipitin bands resulted from cross-reaction of the antiserum with extracts of cells cultured anaerobically with nitrate or anaerobically with ammonia and nitrate. Immunodiffusion experiments with crude bacteroid extract and purified Mo-Fe protein revealed a greater number of precipitin bands in tests conducted under aerobic conditions than those conducted under anaerobic conditions. These results indicate that some of the cross-reactive material observed under aerobic conditions resulted from breakdown of the Mo-Fe protein. Bacteroid extracts of nodules from plants supplied with ammonia exhibited only a trace of nitrogenase activity. The addition of an excess of the Fe protein component of nitrogenase, however, resulted in 270-fold enhancement of activity indication the presence of active Mo-Fe protein in these extracts. Our experiments together with results published elsewhere provide evidence that the genetic information for synthesis of a part of the Mo-Fe component of nitrogenase is carried by Rhizobium.

Ethidium bromide-resistant mutant of Bacillus subtilis

An ethidium bromide-resistant mutant (EB8) derived from a Marburg strain of Bacillus subtilis was found to be conditionally resistant to 10 mug of ethidium bromide per ml. Expression of resistance is complete only during vegetative growth at incubation temperatures above 30 C in complex medium or minimal medium supplemented with Casamino Acids. Strain EB8 is cross-resistant to acriflavine and proflavine. The ethidium bromide resistance marker is co-transduced with hisA1 at a frequency of 6% and is located to the right of hisA1 on the B. subtilis chromosome as it is usually represented on the map. Incorporation of [5-(3)H] uridine by strain EB8 showed that ribonucleic acid synthesis in both whole cells and protoplasts is ethidium bromide-resistant.