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Journal of Bacteriology, February 2009, p. 735-746, Vol. 191, No. 3
0021-9193/09/$08.00+0     doi:10.1128/JB.01404-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

Y4lO of Rhizobium sp. Strain NGR234 Is a Symbiotic Determinant Required for Symbiosome Differentiation{triangledown}

Feng-Juan Yang,# Li-Li Cheng,# Ling Zhang, Wei-Jun Dai, Zhe Liu, Nan Yao, Zhi-Ping Xie, and Christian Staehelin*

State Key Laboratory of Biocontrol, School of Life Sciences, SunYat-Sen (Zhongshan) University, Guangzhou 510275, China

Received 7 October 2008/ Accepted 25 November 2008

Type 3 (T3) effector proteins, secreted by nitrogen-fixing rhizobia with a bacterial T3 secretion system, affect the nodulation of certain host legumes. The open reading frame y4lO of Rhizobium sp. strain NGR234 encodes a protein with sequence similarities to T3 effectors from pathogenic bacteria (the YopJ effector family). Transcription studies showed that the promoter activity of y4lO depended on the transcriptional activator TtsI. Recombinant Y4lO protein expressed in Escherichia coli did not acetylate two representative mitogen-activated protein kinase kinases (human MKK6 and MKK1 from Medicago truncatula), indicating that YopJ-like proteins differ with respect to their substrate specificities. The y4lO gene was mutated in NGR234 (strain NGR{Omega}y4lO) and in NGR{Omega}nopL, a mutant that does not produce the T3 effector NopL (strain NGR{Omega}nopL{Omega}y4lO). When used as inoculants, the symbiotic properties of the mutants differed. Tephrosia vogelii, Phaseolus vulgaris cv. Yudou No. 1, and Vigna unguiculata cv. Sui Qing Dou Jiao formed pink effective nodules with NGR234 and NGR{Omega}nopL{Omega}y4lO. Nodules induced by NGR{Omega}y4lO were first pink but rapidly turned greenish (ineffective nodules), indicating premature senescence. An ultrastructural analysis of the nodules induced by NGR{Omega}y4lO revealed abnormal formation of enlarged infection droplets in ineffective nodules, whereas symbiosomes harboring a single bacteroid were frequently observed in effective nodules induced by NGR234 or NGR{Omega}nopL{Omega}y4lO. It is concluded that Y4lO is a symbiotic determinant involved in the differentiation of symbiosomes. Y4lO mitigated senescence-inducing effects caused by the T3 effector NopL, suggesting synergistic effects for Y4lO and NopL in nitrogen-fixing nodules.

* Corresponding author. Mailing address: State Key Laboratory of Biocontrol, School of Life Sciences, SunYat-Sen (Zhongshan) University, East Campus, Bei San Road, Guangzhou 510006, China. Phone: 8620 39332976. Fax: 8620 39332978. E-mail: cst{at}mail.sysu.edu.cn

{triangledown} Published ahead of print on 5 December 2008.

# F.-J.Y. and L.-L.C. contributed equally to this work.

Journal of Bacteriology, February 2009, p. 735-746, Vol. 191, No. 3
0021-9193/09/$08.00+0     doi:10.1128/JB.01404-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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