## 1 Introduction  
Bacteria need to co-ordinate cellular responses to unfavorable environmental conditions [1]. One major strategy to cope with environmental stress is the activation of stringent response, a global regulatory system [2]. The stringent response is activated by (p)ppGpp (guanosine tetrapphosphate and guanosine pentaphosphate) [3]. The proteins from the ReLA/SpoT (Rsh) family are the key players, synthesizing (p)ppGpp from ATP and either GTP or GDP, and degrading (p)ppGpp to pyrophosphate and either GTP or GDP. Most species in _Betaproteobacteria_ and _Gammaproteobacteria_ contain two multi-domain Rsh enzymes, ReLA and SpoT, while the majority of bacteria contain only a single Rsh protein [3]. A number of studies have demonstrated that Rsh affected the expression of a wide range of genes involved in physiological processes in bacteria such as _Escherichia coli_, _Staphylococcus aureus and _Rhizobium etli_[4; 5; 6]. In _R. etli_, there were 834 genes in the Rsh regulon involved in various cellular processes such as transcriptional regulation, signal transduction, production of sigma factors and non-coding RNAs [6]. Sphingomonads, a group of _Alplaproteobacteria_, are widely distributed in polluted and oligotrophic environments [7; 8]. Sphingomonads have drawn much attention for their traits, including the pronounced abilities of degrading a wide range of recalcitrant natural and xenobiotic compounds such as polycyclic aromatic hydrocarbons (PAH) [7; 9], the substitution of sphingolipids for lipopolysaccharide in their outer membrane [10; 11] as well as their production of exopolysaccharides (EPS) [12; 13]. The genomes of sphingomonads contain one single _rsli_ gene, which responds to environmental stress as in other bacteria [14]. However, to date, no transcriptome-wide study has been conducted to investigate Rsh target genes in any strain of sphingomonads. One of the physiological activities regulated by Rsh is quorum sensing (QS), a mechanism of intercellular communication. In Gram-negative bacteria, the main QS signal is \(N\)-acyl-L-homoserine lactone (AHL), which is produced by LuxI-type synthases. LuxR-type receptors can bind to AHL and then stimulate the expression of _luxI_ homologs [15]. AHL accumulation is dependent on RelA/SpoT homologs in various bacteria such as _Pseudomonas aeruginosa_[16; 17; 18] and _R. etli_[19], and AHL degradation is regulated by (p)ppGpp via AttM in _Agrobacteriumium tumefaciens_[20; 21]. In _P. aeruginosa_, deletion of both _relA_ and _spoT_ resulted in increased levels of 4-hydroxyl-2-heptylquinoline and 3,4-dihydroxy-2-heptylquinoline via up-regulated _pqsA_ and _pqsR_ expression and decreased levels of butanoyll-homoserine lactone and 3-oxo-dodecanoyll-homoserine lactone via down-regulated _rhll_, _rhlR_, _lasI_, and _lasR_ expression [18]. In recent years, an increasing number of strains in sphingomonads which produced AHLs have been isolated [22]. The comparative genomic analyses of 62 sphingomonads genomes showed that the canonical _luxI_/\(R\)-type QS network was widespread within sphingomonads [23]. In the previous study, a Tn5 mutant of _Novosphingobium_ sp. Kr 2-17, deficient in AHL accumulation, was found to have an insertion in an _rsli_ gene, suggesting that QS was under the regulation of Rsh in _Novosphingobium_, a member of sphingomonads. However, the potential molecular mechanism remains unknown. _Novosphingobium pentaromativorans_ US6-1, which has been shown to have potential in aromatic hydrocarbons bioremediation, is a type strain belonging to sphingomonads [24]. Its genome sequencing has been completed and the genome database is accessible from the public NCBI database [25]. In this study, we identified an _rsli_ gene in _N. pentaromativorans_ US6-1 (annotated as _rsh_US6-1) by sequence analysis. The wild-type strain produced ppGpp in static culture medium while _rsh_US6-1 deletion mutant (\(\Delta\)_rsh_) did not.