22
2004). Regulation has been studied in diverse microorganisms including Klebsiella,
Azotobacter, the anaerobes Clostridium and methanogenic Archaea, and unicellular
and filamentous cyanobacteria (Klipp et al. 2004). There are a variety of mechanisms involved, depending on microorganism, but generally they respond to N
availability, O 2 , light, Fe and Mo.
Microorganisms, in general, need to sense N availability and respond with various pathways and transporters to use alternate N sources, for example. Diazotrophs
use the same or similar mechanisms for sensing N status, mostly involving a family
of proteins called PII (Leigh and Dodsworth 2007) which respond to 2-oxoglutarate
and ATP. However, the details of molecules sensed, the targets and their modifications, differ among different phylogenetic groups of microorganisms (Leigh and
Dodsworth 2007). Some microorganisms use GlnD to sense glutamine levels, and
the PII proteins (GlnB and/or GlnK) to sense 2-oxoglutarate and ATP as a measure
of carbon and energy status (Leigh and Dodsworth 2007; Masepohl 2017). In photosynthetic bacteria, an example of the regulatory cascade is shown in Fig. 2.5.
Fig. 2.5 Example of regulation of nitrogenase genes in bacteria showing regulatory cascade
beginning with phosphorylation (yellow) of NtrC (blue) by NtrB (dark blue), leading to transcription of nifA (Mo nitrogenase, green) and anfA (alternative nitrogenase, green) which activates the
nitrogenase operons. If Mo is present MopA (red) represses the synthesis of the alternative anfA
gene and its own operon containing the Mo transporter genes ModABC. Activation by NifA
(green) is blocked by NifL which is expressed when O 2 is present (not shown). Figure derived from
Masepohl (2017)
2 Fundamentals of N 2 Fixation
2004). Regulation has been studied in diverse microorganisms including Klebsiella,
Azotobacter, the anaerobes Clostridium and methanogenic Archaea, and unicellular
and filamentous cyanobacteria (Klipp et al. 2004). There are a variety of mechanisms involved, depending on microorganism, but generally they respond to N
availability, O 2 , light, Fe and Mo.
Microorganisms, in general, need to sense N availability and respond with various pathways and transporters to use alternate N sources, for example. Diazotrophs
use the same or similar mechanisms for sensing N status, mostly involving a family
of proteins called PII (Leigh and Dodsworth 2007) which respond to 2-oxoglutarate
and ATP. However, the details of molecules sensed, the targets and their modifications, differ among different phylogenetic groups of microorganisms (Leigh and
Dodsworth 2007). Some microorganisms use GlnD to sense glutamine levels, and
the PII proteins (GlnB and/or GlnK) to sense 2-oxoglutarate and ATP as a measure
of carbon and energy status (Leigh and Dodsworth 2007; Masepohl 2017). In photosynthetic bacteria, an example of the regulatory cascade is shown in Fig. 2.5.
Fig. 2.5 Example of regulation of nitrogenase genes in bacteria showing regulatory cascade
beginning with phosphorylation (yellow) of NtrC (blue) by NtrB (dark blue), leading to transcription of nifA (Mo nitrogenase, green) and anfA (alternative nitrogenase, green) which activates the
nitrogenase operons. If Mo is present MopA (red) represses the synthesis of the alternative anfA
gene and its own operon containing the Mo transporter genes ModABC. Activation by NifA
(green) is blocked by NifL which is expressed when O 2 is present (not shown). Figure derived from
Masepohl (2017)
2 Fundamentals of N 2 Fixation