Expression Islands Clustered on the Symbiosis Island of the Mesorhizobium loti Genome

Toshiki Uchiumi, Takuji Ohwada, Manabu Itakura, Hisayuki Mitsui, Noriyuki Nukui, Pramod Dawadi, Takakazu Kaneko, Satoshi Tabata, Tadashi Yokoyama, Kouhei Tejima, Kazuhiko Saeki, Hirofumi Omori, Makoto Hayashi, Takaki Maekawa, Rutchadaporn Sriprang, Yoshikatsu Murooka, Shigeyuki Tajima, Kenshiro Simomura, Mika Nomura, Akihiro SuzukiYoshikazu Shimoda, Kouki Sioya, Mikiko Abe, Kiwamu Minamisawa

Research output: Contribution to journalArticlepeer-review

160 Citations (Scopus)


Rhizobia are symbiotic nitrogen-fixing soil bacteria that are associated with host legumes. The establishment of rhizobial symbiosis requires signal exchanges between partners in microaerobic environments that result in mutualism for the two partners. We developed a macroarray for Mesorhizobium loti MAFF303099, a microsymbiont of the model legume Lotus japonicus, and monitored the transcriptional dynamics of the bacterium during symbiosis, microaerobiosis, and starvation. Global transcriptional profiling demonstrated that the clusters of genes within the symbiosis island (611 kb), a transmissible region distinct from other chromosomal regions, are collectively expressed during symbiosis, whereas genes outside the island are downregulated. This finding implies that the huge symbiosis island functions as clustered expression islands to support symbiotic nitrogen fixation. Interestingly, most transposase genes on the symbiosis island were highly upregulated in bacteroids, as were nif, fix, fdx, and rpoN. The genome region containing the fixNOPQ genes outside the symbiosis island was markedly upregulated as another expression island under both microaerobic and symbiotic conditions. The symbiosis profiling data suggested that there was activation of amino acid metabolism, as well as nif-fix gene expression. In contrast, genes for cell wall synthesis, cell division, DNA replication, and flagella were strongly repressed in differentiated bacteroids. A highly upregulated gene in bacteroids, mlr5932 (encoding 1-aminocyclopropane-1-carboxylate deaminase), was disrupted and was confirmed to be involved in nodulation enhancement, indicating that disruption of highly expressed genes is a useful strategy for exploring novel gene functions in symbiosis.

Original languageEnglish
Pages (from-to)2439-2448
Number of pages10
JournalJournal of bacteriology
Issue number8
Publication statusPublished - 2004 Apr

ASJC Scopus subject areas

  • Microbiology
  • Molecular Biology


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