期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：49
页码：15244-15249
DOI：10.1073/pnas.1417797112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe agriculturally important symbiosis between nitrogen-fixing bacteria (rhizobia) and their legume hosts occurs within root nodules. This partnership requires a molecular dialogue that ensures specificity and directs the codevelopment of the two organisms during nodule formation. This paper characterizes a protein, host range restriction peptidase (HrrP), which plays a role in this dialogue. Rhizobial strains that express HrrP tend to exhibit more parasitic properties, such as failing to provide fixed nitrogen for their hosts and proliferating more abundantly within nodule tissue. HrrP likely exhibits these properties by actively degrading plant-derived chemical signals that normally stimulate symbiotic cooperation. Legume-rhizobium pairs are often observed that produce symbiotic root nodules but fail to fix nitrogen. Using the Sinorhizobium meliloti and Medicago truncatula symbiotic system, we previously described several naturally occurring accessory plasmids capable of disrupting the late stages of nodule development while enhancing bacterial proliferation within the nodule. We report here that host range restriction peptidase (hrrP), a gene found on one of these plasmids, is capable of conferring both these properties. hrrP encodes an M16A family metallopeptidase whose catalytic activity is required for these symbiotic effects. The ability of hrrP to suppress nitrogen fixation is conditioned upon the genotypes of both the host plant and the hrrP-expressing rhizobial strain, suggesting its involvement in symbiotic communication. Purified HrrP protein is capable of degrading a range of nodule-specific cysteine-rich (NCR) peptides encoded by M. truncatula. NCR peptides are crucial signals used by M. truncatula for inducing and maintaining rhizobial differentiation within nodules, as demonstrated in the accompanying article [Horvath B, et al. (2015) Proc Natl Acad Sci USA, 10.1073/pnas.1500777112]. The expression pattern of hrrP and its effects on rhizobial morphology are consistent with the NCR peptide cleavage model. This work points to a symbiotic dialogue involving a complex ensemble of host-derived signaling peptides and bacterial modifier enzymes capable of adjusting signal strength, sometimes with exploitative outcomes.
