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Appl Environ Microbiol. 1986 March; 51(3): 457-461
Copyright © 1986, American Society for Microbiology. All Rights Reserved.

Rhizobins, a Group of Peptides in the Free-Amino-Acid Pool of the Soybean-Rhizobium System

Andrew S. Garay, Joy A. Ahlgren^*, Mark A. Gonzalez, Mark A. Stasney and Paul C. Madtes

Department of Biochemistry and Biophysics, Texas Agricultural Experiment Station, Texas A&M University, College Station, Texas 77843

ABSTRACT

Free-living Rhizobium (according to Bergey's Manual of Systematic Bacteriology, [1984, The Williams & Wilkins Co., Baltimore], Bradyrhizobium) japonicum was found to release a peptide into the nutrient media. Soybean nodules contained this peptide and exuded it into the soil. The name "rhizobin A" is suggested for this peptide. Nodules also contained another peptide, rhizobin B, as well as an unidentified, ninhydrin-positive compound, rhizobin C. The three peptides were confined to the free-amino-acid pool of the soluble fraction and eluted consecutively from a cation-exchange column. Rhizobin A was isolated in a highly purified form; its molecular mass was approximately 1,600 daltons as determined by Sephadex gel filtration and mass spectrometry. The amino-acid composition could be determined only approximately, because a long time was necessary for acid hydrolysis, possibly due to unusual linkages. The rhizobin concentration in soybean nodules continually increased during 50 days of growth, from 2 to approximately 400 µg/g (fresh weight). When combined nitrogen was added to nodulated soybean and subsequently removed, nitrogenase activity, nodulation, and nodule growth first decreased and then recovered. The relative amount of rhizobin A followed a similar pattern. Rhizobins were not detected in the roots, stems, and leaves of nodulated soybean plants. They were present in Lupinus nodules, but absent in alder nodules.

^* Corresponding author.

Present address: Department of Biology and Chemistry, Point Loma Nazarene College, 3900 Loma Land Drive, San Diego, CA 92106.

Journal paper no. TA-20210 of the Texas Agricultural Experiment Station.