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Title: Functional nodFE genes are present in Sinorhizobium sp. strain MUS10, a symbiont of tropical legume Sesbania rostrata

item Krishnan, Hari

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2008
Publication Date: 3/7/2008
Citation: Krishnan, H.B., Chronis, D. 2008. Functional nodFE genes are present in Sinorhizobium sp. strain MUS10, a symbiont of tropical legume Sesbania rostrata. Applied and Environmental Microbiology. 74:2921-2923.

Interpretive Summary: Rhizobia are soil-dwelling bacteria that form nodules on the roots of legume plants. The nodules are specialized structures where atmospheric nitrogen is fixed by the bacterium, which in turn, is utilized by soybean plants for growth and development. This process is termed biological nitrogen fixation and it enables soybean plants to grow in nitrogen-poor soils. Nodule development involves the exchange of a series of molecular signals between plants and bacteria. Elucidation of the intricate signal exchange process between the plants and rhizobia is crucial to understanding the initial events in the nodulation process. In this study, we have determined the nucleotide sequences of nodFE genes which play an important role in the synthesis of Nod factors, an important signal molecule produced by rhizobia. Information obtained from this basic study will help scientists understand the factors that limit the formation of nitrogen-fixing nodules. Such information should enable scientists to manipulate biological nitrogen fixation facilitating an increase in legume yields with minimal use of nitrogen fertilizers.

Technical Abstract: Sinorhizobium sp. strain MUS10, a rhizobium from the Indian subcontinent, forms nitrogen-fixing nodules on the stems and roots of tropical legume Sesbania rostrata. The structure of Nod factors (NFs) of MUS10 are similar to those of Azorhizobium caulinodans, S. saheli bv sesbaniae and S. terangae bv sesbaniae, strains originating from Africa. In addition to producing characteristic arabinosylated and fucosylated Nod factors, this Indian strain also elaborates NFs with N-linked fatty acids containing a delta-hydroxy group or with one carbonyl-conjugated double bond. The biosynthesis of fatty acids carrying trans double bonds conjugated to the carbonyl group requires functional nod FE genes which have not been detected in the Sesbania-nodulating strains. To investigate the existence of nodF in Sesbania-nodulating strains, we performed Southern blot analysis using nodF of R. leguminosarum bv. viciae as a probe. The presence of nodF homologous sequences was detected in S. saheli bv sesbaniae, S. terangae bv sesbaniae, and MUS10 but not in A. caulinodans. We have cloned the nodFE operon from MUS10 and determined their nucleotide sequences. The nodFE of MUS10 encodes proteins of 10 and 42 kDa, respectively. Nod-box-like sequences were located upstream of nodF. MUS10 NodF, which shows sequence homology to acyl-carrier protein from rhizobia, posses a consensus sequence (-LGXDSL-) that is required for the attachment of a phosphopantetgeine prosthetic group. Our results demonstrate for the first time the occurrence of nodFE in a symbiont that nodulates a legume host not belonging to Galegoid group.