Background Bacterial communication is involved in regulation of cellular mechanisms such as metabolic processes, microbe-host interactions or biofilm formation. In the nitrogen-fixing model endophyte of grasses Azoarcus sp. strain BH72, known cell-cell signaling systems have not been identified; however, the pilA gene encoding the structural protein of type IV pili that are essential for plant colonization appears to be regulated in a population density-dependent manner. Methodology/Principal Findings Our data suggest that pilAB expression is affected by population density, independent of autoinducers typical for Gram-negative bacteria, likely depending on unknown secreted molecule(s) that can be produced by different bacterial species. We used transcriptomic and proteomic approaches to identify target genes and proteins differentially regulated in conditioned supernatants in comparison to standard growth conditions. Around 8% of the 3992 protein-coding genes of Azoarcus sp. and 18% of the detected proteins were differentially regulated. Regulatory proteins and transcription factors among the regulated proteins indicated a complex hierarchy. Differentially regulated genes and proteins were involved in processes such as type IV pili formation and regulation, metal and nutrient transport, energy metabolism, and unknown functions mediated by hypothetical proteins. Four of the newly discovered target genes were further analyzed and in general they showed regulation patterns similar to pilAB. The expression of one of them was shown to be induced in plant roots. Conclusion/Significance This study is the first global approach to initiate characterization of cell density-dependent gene regulation mediated by soluble molecule(s) in the model endophyte Azoarcus sp. strain BH72. Our data suggest that the putative signaling molecule(s) are also produced by other Proteobacteria and might thus be used for interspecies communication. This study provides the foundation for the development of robust reporter systems for Azoarcus sp. to analyze mechanisms and molecules involved in the population-dependent gene expression in this endophyte in future.

The FEBS JournalVolume 274, Issue s1 p. 6-17 Free Access Symposia Lectures First published: 16 July 2007 https://doi.org/10.1111/j.0014-2956.2007.05866_1.xCitations: 1AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume274, Issues1July 2007Pages 6-17 RelatedInformation

Nature Biotechnology 24, 1385–1391 (2006); published online 22 October 2006; corrected after print 26 March 2007 In the version of this article initially published, the gene name nosRZDYFLX on p. 1388, left column, line 5, is incorrect; the correct name is nosZRDFYL. The error has been corrected in the PDF version of the article.

Nature Biotechnology 24, 1385–1391 (2006); published online 22 October 2006; corrected after print 26 March 2007 In the version of this article initially published, the gene name nosRZDYFLX on p. 1388, left column, line 5, is incorrect; the correct name is nosZRDFYL. The error has been corrected in the PDF version of the article.

Azoarcus sp. strain BH72, a mutualistic endophyte of rice and other grasses, is of agrobiotechnological interest because it supplies biologically fixed nitrogen to its host and colonizes plants in remarkably high numbers without eliciting disease symptoms. The complete genome sequence is 4,376,040-bp long and contains 3,992 predicted protein-coding sequences. Genome comparison with the Azoarcus-related soil bacterium strain EbN1 revealed a surprisingly low degree of synteny. Coding sequences involved in the synthesis of surface components potentially important for plant-microbe interactions were more closely related to those of plant-associated bacteria. Strain BH72 appears to be 'disarmed' compared to plant pathogens, having only a few enzymes that degrade plant cell walls; it lacks type III and IV secretion systems, related toxins and an N-acyl homoserine lactones–based communication system. The genome contains remarkably few mobile elements, indicating a low rate of recent gene transfer that is presumably due to adaptation to a stable, low-stress microenvironment.