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The genetics of N 2 fixation in symbioses are more complex than free-living N 2 -
fixing microorganisms and requires additional genes, regulation of gene expression
and molecular signalling (Roy et al. 2019). Interactions involving both partners
affect each other’s gene expression that leads to facilitation of invasion of tissue and
cells, and formation of symbiosomes containing bacteroids (Roy et al. 2019). This
complex process which has been determined largely by genetic methods is beyond
Fig. 2.4 Major genes involved in N 2 fixation (a) and organization of major nif gene cluster in
diverse microorganisms, including dominant marine N 2 fixers (b). In (a) the schematic shows the
complex organization and number of genes associated with the capacity for N 2 fixation in the freeliving organism Azotobacter vinelandii. All three known N 2 fixation systems; molybdenumdependent, vanadium-dependent, and Fe-only are encoded in the A. vinelandii genome. Catalytic
components (dinitrogenase reductase and dinitrogenase) for each system are indicated by solid
blue. Genes whose products are required for activation of all three systems are indicated by red
dots above the relevant gene. Transcription units are indicated by arrows below gene clusters.
Various gene products that do not include the catalytic units are those involved in metallocluster
assembly, regulation, O 2 protection, and electron transfer. Genes indicated as nif, naf, rnf, nfa, fix,
vnf, and anf are trivial designations assigned by researchers that initially discovered them.
(Courtesy of Dennis Dean)
2.4 Non-structural nif Genes and Protein Assembly
The genetics of N 2 fixation in symbioses are more complex than free-living N 2 -
fixing microorganisms and requires additional genes, regulation of gene expression
and molecular signalling (Roy et al. 2019). Interactions involving both partners
affect each other’s gene expression that leads to facilitation of invasion of tissue and
cells, and formation of symbiosomes containing bacteroids (Roy et al. 2019). This
complex process which has been determined largely by genetic methods is beyond
Fig. 2.4 Major genes involved in N 2 fixation (a) and organization of major nif gene cluster in
diverse microorganisms, including dominant marine N 2 fixers (b). In (a) the schematic shows the
complex organization and number of genes associated with the capacity for N 2 fixation in the freeliving organism Azotobacter vinelandii. All three known N 2 fixation systems; molybdenumdependent, vanadium-dependent, and Fe-only are encoded in the A. vinelandii genome. Catalytic
components (dinitrogenase reductase and dinitrogenase) for each system are indicated by solid
blue. Genes whose products are required for activation of all three systems are indicated by red
dots above the relevant gene. Transcription units are indicated by arrows below gene clusters.
Various gene products that do not include the catalytic units are those involved in metallocluster
assembly, regulation, O 2 protection, and electron transfer. Genes indicated as nif, naf, rnf, nfa, fix,
vnf, and anf are trivial designations assigned by researchers that initially discovered them.
(Courtesy of Dennis Dean)
2.4 Non-structural nif Genes and Protein Assembly