Journal Article

Requirement for <i>Mesorhizobium loti</i> Ornithine Transcarbamoylase for Successful Symbiosis with <i>Lotus japonicus</i> as Revealed by an Unexpected Long-Range Genome Deletion

Elina Mishima, Atsuko Hosokawa, Haruko Imaizumi-Anraku, Katsuharu Saito, Masayoshi Kawaguchi and Kazuhiko Saeki

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 49, issue 3, pages 301-313
Published in print March 2008 | ISSN: 0032-0781
Published online March 2008 | e-ISSN: 1471-9053 | DOI: https://dx.doi.org/10.1093/pcp/pcn004
Requirement for Mesorhizobium loti Ornithine Transcarbamoylase for Successful Symbiosis with Lotus japonicus as Revealed by an Unexpected Long-Range Genome Deletion

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With the original aim of surveying the role of exopolysaccharide (EPS) in Lotus–Mesorhizobium symbiosis, we carried out Tn5 mutagenesis of Mesorhizobium loti and obtained 32 mutants with defects in EPS biosynthesis. One of the mutants, HIA22, formed pseudonodules and failed to fix nitrogen with Lotus japonicus. However, complementation analysis unexpectedly revealed that the potential gene with the locus tag, mll2073, interrupted by Tn5 was responsible for neither normal EPS synthesis nor symbiosis. Further analysis uncovered that HIA22 had a genome deletion of approximately 20 kbp, resulting in the loss of two separate genes responsible for EPS biosynthesis and symbiosis. One gene with the locus tag, mll5669, was needed to synthesize normal EPS that fluoresced on medium containing Calcofluor and encoded a homolog of O-antigen acetyl transferase in Salmonella typhimurium. A specific mutant of mll5669, EMB-B58, successfully fixed nitrogen when infected onto L. japonicus. Another gene, mlr5647, was needed to establish fully functional nodules and encoded ornithine carbamoyl transferase [ArgF (EC 2.1.3.3)], which participates in arginine biosynthesis. A specific mutant of mlr5647, EMB-Y2, showed arginine auxotrophy and formed infection threads, but the nodules formed by this strain had few infected cells filled with bacteroids. These mutant phenotypes were complemented by supplementation of arginine or citrulline to bacterial or plant medium. EMB-Y2 represented a novel class of rhizobial arginine auxotrophs with symbiotic deficiency, and its phenotypes indicated that sufficient supply of citrulline or its derivative is essential for successful infection or for a stage in the infection process in Lotus–Mesorhizobium symbiosis.

Keywords: Arginine auxotroph; Exopolysaccharide; Lotus japonicus; Mesorhizobium loti; Nitrogen fixation; Symbiosis

Journal Article.  7614 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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