Nol A Represses nod Gene Expression in Bradyrhizobium japonic urn. Thomas C. Dockendorff. Center for Legume Research, Department of Microbiology and Graduate Program of Ecology, The University of Tennessee, Knoxville, TN 37996-0845, U.S.A. Juan Sanjuan (1), Philipp Grob(2), and Gary Stacey(1). (1) Center for Legume Research, Department of Microbiology and Graduate Program of Ecology, The University of Tennessee, Knoxville, TN 37996-0845, U.S.A., (2) Mikrobiologisches Institut, Eidgenossiche Technische Hochschule, ETH-Zentrum, CH-8092 Zurich, Switzerland. MPMI 7:596-602. Accepted 9 June 1994. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1994.

Studies of lacZ fusions in mutant strains of Bradyrhizobium japonicum strain USDA110 show that the nodD1 and nodYABCSUU operons arc expressed via NodW and isoflavones in the absence of NodD, and NodD2 when the adjacent nolA gene is deleted. Such a result suggests that NolA exerts a repressive effect on isoflavone-induced expression of the nodD, and nodYABCSUU operons. Indeed, conjugation of a plasmid encoding NolA into wild-type or mutant strains resulted in a marked reduction in nod gene expression. The nolYZ operon, whose regulation is also dependent on NodD, and NodW, is not expressed at a high level in the absence of NolA. These results indicate differential expression of nodulation operons by the action of NodD,, NodW, and NolA. B. japonicum strains deleted for nodD2, nodD2, and nolA nodulatc soybean very poorly and only after a substantial delay. Mobilization of a plasmid encoding nolA into such strains significantly improved nodulation. Fine-tuning of nod gene expression mediated by the interplay of positive regulators (i.e., NodD, and NodW) and negative regulators (i.e., NolA) appears to be important for efficient nodulation by B. japonicum.