## 3 Results and Discussion  
### The Genes Associated with Luminescence in P. phosphoreum and V. qinghaiensis  
As for QS-controlled luminescent bacteria, the regulatory elements in gene cluster are essential for light production. Therefore, we represented the arrangement of bacterial luminescence genes, including the gene upstream of the core light genes (_lux_CDABEG) in _V. fischeri, P. phosphoreum_ and _V. qinghaiensis_ (Figure 1). The core light genes (_lux_CDABEG) are involved in all three strains. The _lux_A and _lux_B encoded two subunits of luciferase, constituting the basis of bacterial luciferase; the _lux_C, _lux_D, _lux_E were responsible for the conversion process of long-chain fatty acids to fatty aldehydes, encoding the fatty acid reductase, transferase and synthase, respectively [15]. The functional _lux_G is considered the source of reduced flavins for a catalytic process [16].  
Compared to the luminescence (_lux_) genes among the three strains, one significant difference is the genes upstream of the core light genes (_lux_CDABEG) (Figure 1). The upstream gene in the opposite direction of _Vibrio fischeri_ is _lux_R, which forms the _lux_I/_lux_R regulatory system with _lux_I (Table 1). The LuxI/LuxR system mainly regulates the _lux_ operon (_lux_CDABEG) expression, which contains the genes necessary for light production in _V. fischeri_. Compared with _V. fischeri_, the regulatory mechanisms of bioluminescence are not well clarified in _P. phosphoreum_ and _V. qinghaiensis_. Instead of _lux_I and _lux_R, _lux_L (also named _lum_P in _P. leiognathi_, Table 1) is located upstream of the _lux_ operon and transcribed in the opposite direction in _P. phosphoreum_. It has been found that lumazine protein encoded by _lux_L plays a role in light emission processes involved in wavelength and intensity transformation. Its derivatives are direct precursors of riboflavin [17]. Compared with the lumininant operon of _V. fischeri_, _V. qinghaiensis_ sp.-Q67 possessed a gene named CCZ37-17825 located in the opposite direction upstream of _lux_C, whose function is still unknown [18]. Furthermore, _Photobacterium_ possesses _lux_F and _rib_ genes absent from the _lux_ operon of _Vibrio_ species. It is believed that _lux_F encoded non-fluorescent flavin proteins which could bind with the bioluminescence inhibitor myristylated FMN to increase the luminescent intensity indirectly [19]. That may be one reason why _P. phosphoreum_ is the brightest species among all the reported bioluminescent bacteria [20]. The additional _rib_ genes (_rib_EBHA) are related to the redox process of FMNH\({}_{2}\), involved in the transformation and synthesis of riboflavin. Few studies have reported the presence of QS-associated genes in _P. phosphoreum_, and the relationship between bioluminescence and QS has not been clear in this species until now. Compared with _V. fischeri_, the _lux_I gene is missing in strain Q67, which is generally recognized as a crucial gene to encode the autoinducer synthase in _V. fischeri_.  
Based on the above description, the present lumininant regulatory genes in _P. phosphoreum_ and _V. qinghaiensis_ differed from that in _V. fischeri_ on both arrangement and nucleotide acid sequences. These differences among the genes associated with the bioluminescence might indicate the presence of different QS systems other than the typical LuxI/LuxR in those two luminescent bacteria. Thus, the following experiments investigated the relationship between bioluminescence and _lux_ gene expression in _P. phosphoreum_ and _V. qinghaiensis_.