Affiliation:
1. Agricultural University of Athens, Department of Agricultural Biotechnology, Iera Odos 75, 118 55 Athens, Greece
2. Albert-Ludwigs University of Freiburg, Chair of Tree Physiology, Georges-Köhler-Allee, Geb. 053/054, 79110 Freiburg i. Br., Germany
Abstract
ABSTRACT
Carbonic anhydrase (CA) (EC 4.2.1.1) is a widespread enzyme catalyzing the reversible hydration of CO
2
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 α-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 α-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 δ
13
C 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 NH
3
protonation, thus facilitating its diffusion and transport to the plant. In addition, changes in the nodule δ
13
C abundance suggest the recycling of at least part of the HCO
3
−
produced by CAA1.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
18 articles.
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