Biochemistry and genetics of Nitrogen fixation in Rhizobium K R.pptx

Biochemistry and genetics of Nitrogen fixation in Rhizobium K R.pptx

1. Biochemistry and genetics of Nitrogen fixation in Rhizobium K R MICRO NOTES 1

2. Introduction Nitrogen fixation is the process by which nitrogen is taken from its molecular form (N 2 ) in the atmosphere and converted into nitrogen compounds useful for other biochemical processes. The nitrogen fixation is carried out by enzyme nitrogenase which are found in microbes Nitrogen fixing bacteria are free living as well as symbiotic Rhizobium is a symbiotic bacteria present in the root nodules of the leguminous plants. K R MICRO NOTES 2

3. Nitrogenase Enzyme K R MICRO NOTES 3

4. Biochemistry of N 2 Fixation K R MICRO NOTES 4

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6. Overall Reaction N 2  + 8H +  + 8e −  + 16 ATP → 2NH 3  + H 2  + 16ADP + 16 P i Energy consumption 12-16 ATP molecules are required per molecule of N 2 to convert into 2NH 3 K R MICRO NOTES 6

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8. Biochemistry of N 2 fixation in root nodules Bacteroids are the site of biochemical pathways taking place during N 2 – fixation in the root nodules. The bacteroids are totally dependent on the plant for the energy requirement and contain the nitrogenase enzyme complex within them instead of releasing it into the plant cell cytoplasm. The major organic compounds transported across the membrane of symbiosome and into the bacteriod proper and intermediates of citric acid cycle particularly the succinic acid, malic acid, and fumaric acid . These acids are used as electron donors for ATP production and are the ultimate source of electrons for the reduction of N 2 to NH 3 , following their conversion to pyruvic acid. K R MICRO NOTES 8

9. Once molecular nitrogen (N 2 ) is reduced to ammonia (NH 3 ), the first stable product of N 2 – fixation, the NH 3 is assimilated into organic nitrogenous compounds. The assimilation of ammonia into organic nitrogenous compounds in the root nodule is carried out primarily by the plant. The low levels of ammonia- assimilatory enzymes in bacteroids, which can assimilate some ammonia diffuses out of the symbiosome into the legume cell cytoplasm and is assimilated therein. The main route of NH 3 assimilation appears to be synthesis of amino acid glutamine by the enzyme glutamine synthetase - glutamate synthase enzyme system. Besides glutamine amino acid, amino acid amides- aspargine and 4- methyl glutamine and the ureides – Allantoin and allantoic acid are synthesized by the legume and subsequently transports to various plant tissues. K R MICRO NOTES 9

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12. Genetics of N 2 Fixation Nif gene K R MICRO NOTES 12

13. ntr gene Nitrogen regulation genes Present in Rhizobium leguminosarum. K R MICRO NOTES 13

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15. Nitrogen Fixing Gene in Rhizobium Genes responsible for nitrogen fixation are called nitrogen fixing genes or Nif genes . If the bacteria possess this capability they are designated as Nif + otherwise Nif -. Nif gene is a cluster of gene highly conserved through evolution and it act as single transferable unit. There are at least 18 bacterial genes responsible for N 2 fixation in this cluster. Symbiotic bacteria of the genus  Rhizobium  synthesize lipo- chito oligosaccharides, called Nod factors (NFs), which act as morphogenic signal molecules on legume hosts. The common  nod ABC  genes, present in all  Rhizobium  species, are required for the synthesis of the core structure of NFs. K R MICRO NOTES 15

16. The genetic map of Rhizobium It is the circular chromosome that can be divided into 100 unit. At the 10 th unit the lac gene is present and tryptone gene at the 30 th unit and Nif is located in the 40 th unit . It is sandwiched between 20 genes of histidine and shikimic acid . Rhizobium meliloti possess the biggest plasmid 1200-1500kb. In nature Nif gene transfer from one cell to another cell occurs through transduction, conjugation or transformation . K R MICRO NOTES 16

17. Nod genes Genes responsible for nodulation are designated as Nod A, Nod B, Nod C, Nod E, Nod F, Nod G, Nod H and Nod Lm. Out of these, Nod A,B and C are genes against nodulation and Nod E,F,G,H and Lm are responsible for root hair infection . Legume genes are also responsible for nodulation and are of 2 kinds. 1)Nod D 75 2) Nod 26 K R MICRO NOTES 17

18. Nod D 75 is responsible for a production of glycoprotein that is synthesized and enclosed in a infection thread. Nod 26 gives rise to a protein located in the peribacteriod membrane. This protein regulates the exchange of nutrients between the bacteriod zone and the outer zone. K R MICRO NOTES 18

19. References S.B. Sullia, S. Shantharam, General Microbiology, 2 nd edition, Oxford & IBH publication, 2006, New Delhi. R.R. Mishra, Soil Microbiology, 1 st edition, CBS Publishers,1996, New Delhi. G. Ray Noggle, George J. Fritz, Introductory plant physiology, Prentice- Hall of India, 2002, New Delhi. N.S. Subba Rao, Soil Microbiology, 4 th edition, Oxford & IBH Publisher, 2015, New Delhi. Robert L. Tate III, Soil Microbiology, 2 nd edition, Wiley India Pvt. Ltd, 2013, New Delhi. Uma Bhardwaj, Biochemistry for Life sciences, 1 st edition, Dorling Kindersley , 2014, New Delhi. Dr. R.P. Singh, Microbiology, 4 th edition, Kalyani Publishers, 2018, Noida (U.P) Images credit- www. Researchgate.com The common  nod ABC  genes of  Rhizobium meliloti  are host-range  determinants-Philippe Roche, 1996 K R MICRO NOTES 19