## 3 Results and Discussion
### Similarities in Cell-Density-Dependent Control of Bioluminescence between T3 and Q67  
The _P. phosphoreum_ T3 and _V. qinghaiensis_ sp.-Q67 were cultured to measure the growth and luminescence over time, respectively (Figure 2a,b). The growth and luminescence \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Bacterium** & \begin{tabular}{c} **Gene Upstream the** \\ _lux_CDAB(_F_EG **Operon** \\ \end{tabular} & \begin{tabular}{c} **Location** \\ **in Genomes** \\ \end{tabular} & \
\begin{tabular}{c} **NCBI** \\ **Accession Number** \\ \end{tabular} & **Fragment Size (nt)** &
\begin{tabular}{c} **EncoProtein** \\ **Length (aa)** \\ \end{tabular} \\ \hline _V. fischeri_ & _lux_I & 1048783-1049355 & HQ436485 & 573 & 190 \\ _P. phosphoreum_ & _lux_R & 1049571-1050323 & L21989 & 753 & 250 \\ _V. qinghaiensis_ & _lux_L & 1127275-1127847 & CP202724 REGION: & 573 & 478 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the gene associated with luminescence in three species. Figure 1: The arrangement of bacterial luminescence (_lux_) genes in three bacteria. Comparison of the size and order of _lux_ genes in three bacteria using diverse colour arrangements to highlight commonalities and differences. curves of the two strains had obvious lag time. After cultivation of T3 and Q67 for about 3 h and 4 h, respectively, the growth of both strains entered the exponential phase, and the luminescence began to increase accordingly. Compared with Q67, the luminescent curve of T3 showed the maximum luminescence intensity increased by two orders of magnitude, reaching 1010. Based on Figure 2a,b, the variation of emitted light intensity (RLU) with cell density (OD) was plotted in Figure 2c,d, respectively. The OD-RLU diagrams of the two strains showed similar variation patterns. The RLU decreased with OD until the OD reached a specific value of 0.10 and 0.46 in T3 and Q67, respectively. After the OD increased above a specific value, the RLU increased. The luminescence intensity reached a fixed order of magnitude when the OD of T3 reached 0.52, and that of Q67 was 1.08. The results indicated that the bioluminescence of both strains is associated with thresholds, below which light would not be emitted. Furthermore, this variation pattern of the OD-RLU curves of both T3 and Q67 is consistent with that of _V.fischeri_, in which the bioluminescence was controlled by QS systems. Thus, the above results suggested that bioluminescence may be controlled by QS in both strains, as in _V.fischeri_[21]. Figure 2: bioluminescence and _lux_ gene-expression levels of T3 and Q67 throughout the growth phases. (**a,b**) Growth and bioluminescence kinetics of T3 and Q67. Red arrows represent the sampling times S1 to S6. (**c,d**) Light emission of T3 and Q67 according to cell density. (**e,f**) Expression level of the _lux_ DAB(F)E genes in T3 and Q67 for sampling times S1–S6. To further demonstrate the luminescence behaviour, the luminescent gene expression level on diverse growth phases of T3 and Q67 was monitored by RT-PCR (Figure 2e,f). As for strain T3, the expression level of selected _lux_ gene, including _lux_B, _lux_F, _lux_E, _lux_D and _lux_A, in the exponential growth phase (S2, S3 and S4) was much higher than that in the lag and stationary phase (S1, S5 and S6). The _lux_B encoding beta subunit of luciferase exhibited a significant variation range in the expression level of T3, which dramatically raised by 44-fold in S3 than that in S1. One of the potential explanations for this result was that luciferase-encoding gene _lux_B played a crucial role in the bioluminescence of T3, causing the higher luminescence intensity of T3 (10\({}^{3}\)-10\({}^{4}\) fold) than that of Q67 at the same cell density (Figure 2a,b). Besides that, it is worth mentioning that _lux_F, which was considered to originate from the ancestral _lux_B gene in _Photobacterium_, also showed a higher relative expression level, with its role as a scavenger for myrFMN to inhibit luciferase [22; 23].