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Journal of Bacteriology, April 2009, p. 2593-2600, Vol. 191, No. 8
0021-9193/09/$08.00+0     doi:10.1128/JB.01456-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Characterization of a Mesorhizobium loti {alpha}-Type Carbonic Anhydrase and Its Role in Symbiotic Nitrogen Fixation{triangledown}

Chrysanthi Kalloniati,1 Daniela Tsikou,1 Vasiliki Lampiri,1 Mariangela N. Fotelli,1 Heinz Rennenberg,2 Iordanis Chatzipavlidis,1 Costas Fasseas,1 Panagiotis Katinakis,1 and Emmanouil Flemetakis1*

Agricultural University of Athens, Department of Agricultural Biotechnology, Iera Odos 75, 118 55 Athens, Greece,1 Albert-Ludwigs University of Freiburg, Chair of Tree Physiology, Georges-Köhler-Allee, Geb. 053/054, 79110 Freiburg i. Br., Germany2

Received 16 October 2008/ Accepted 13 January 2009

Carbonic anhydrase (CA) (EC 4.2.1.1) is a widespread enzyme catalyzing the reversible hydration of CO2 to bicarbonate, a reaction that participates in many biochemical and physiological processes. Mesorhizobium loti, the microsymbiont of the model legume Lotus japonicus, possesses on the symbiosis island a gene (msi040) encoding an {alpha}-type CA homologue, annotated as CAA1. In the present work, the CAA1 open reading frame from M. loti strain R7A was cloned, expressed, and biochemically characterized, and it was proven to be an active {alpha}-CA. The biochemical and physiological roles of the CAA1 gene in free-living and symbiotic rhizobia were examined by using an M. loti R7A disruption mutant strain. Our analysis revealed that CAA1 is expressed in both nitrogen-fixing bacteroids and free-living bacteria during growth in batch cultures, where gene expression was induced by increased medium pH. L. japonicus plants inoculated with the CAA1 mutant strain showed no differences in top-plant traits and nutritional status but consistently formed a higher number of nodules exhibiting higher fresh weight, N content, nitrogenase activity, and {delta}13C abundance. Based on these results, we propose that although CAA1 is not essential for nodule development and symbiotic nitrogen fixation, it may participate in an auxiliary mechanism that buffers the bacteroid periplasm, creating an environment favorable for NH3 protonation, thus facilitating its diffusion and transport to the plant. In addition, changes in the nodule {delta}13C abundance suggest the recycling of at least part of the HCO3– produced by CAA1.

* Corresponding author. Mailing address: Department of Agricultural Biotechnology, Agricultural University of Athens, Iera Odos 75, 118 55 Athens, Greece. Phone: (30) 210 5294343. Fax: (30) 210 5294314. E-mail: mflem{at}aua.gr

{triangledown} Published ahead of print on 13 February 2009.

Journal of Bacteriology, April 2009, p. 2593-2600, Vol. 191, No. 8
0021-9193/09/$08.00+0     doi:10.1128/JB.01456-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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