# Transcription of _Listeria monocytogenes_ Key Virulence Genes on Tomato, Cucumber and Carrot
## 4 Discussion
In addition, relative transcription of both genes seemed to be unaffected by substrate and temperature, with a few notable exceptions. Among them, downregulation of _sigB_ was observed on tomato at all temperatures. However, this was accompanied by up- and down- regulation of _prfA_ at 4 \({}^{\circ}\)C (at different sampling times) and upregulation at 30 \({}^{\circ}\)C. Downregulation of _prfA_ could be explained through the effect of the \(\sigma^{\text{B}}\)-dependent promoter P2\({}_{prfA}\), while upregulation through the effect of the \(\sigma^{\text{B}}\)-independent promoter P1\({}_{prfA}\). The bicistronic _plCA-prfA_ mRNA, which is initiated by the _plCA_ promoter, should be absent at temperatures below 37 \({}^{\circ}\)C [49], i.e., the temperatures employed in the present study. This seems to be partially verified, considering the only moderate correlation between _prfA_ and _plCA_, which was indicated by Pearson correlation coefficient. Internalins InIA, InIB, InIC and InIJ are necessary for effective invasion. Indeed, deletion mutants exhibited significantly attenuated virulence capacity [5; 50; 51; 52]. In the present study, based on the times that log\({}_{2}\)(FC) value indicated lack of regulation, the relative transcription of _inIA_, _inIB_, _inIC_ and _inIJ_ was generally not affected by substrate and temperature. However, in several cases, effect of substrate and temperature on internalin genes relative transcription was observed. The substrate mostly affected _inIA_, _inIB_ and _inIC_ relative transcription and only marginally the one of _inI_J. Subsistence on tomato affected _inIA_ and _inIC_ relative transcription more than subsistence on cucumber did. On the contrary, cucumber affected _inIB_ relative transcription more than tomato did. In nearly all cases, subsistence on cucumber resulted in downregulation, while subsistence on tomato in upregulation; carrot had only marginal effect. Regarding the effect of temperature, this was more pronounced on tomato and cucumber and the genes mostly affected were _inIA_, _inIB_ and _inIC_ in the first case, and _inI_C and _inIJ_ in the second. All the above suggest that transcription regulation of _inIA_, _inIB_ and _inIC_ is related. Indeed, _inIA_ and _inIB_ form the operon _inIAB_, the transcription of which is controlled by both \(\sigma^{\text{B}}\) and PrfA; transcription of _inIC_ is controlled by PrfA, while transcription of _inI_ by none of the above [53]. The effect of _prfA_ on the relative transcription of _inIA_, _inIB_ and _inIC_ was also verified by the high (above 0.7) Pearson correlation coefficient. On the contrary, only a moderate correlation was exhibited between _prfA_ and _inI_ as well as between _sigB_ and all four internalins. The role of _hly_ in virulence has been adequately highlighted; it encodes for (LLO), the absence of which has been reported to result in a virulence [54; 55]. However, [PERSON] et al. [52] reported that LLO-dependent internalization did not occur in the case of cytotrophoblasts and endothelial cells, which indicates that LLO deficient strains avirulence is tissue dependent. Relative transcription of _hly_ seemed to be substantially affected by substrate and temperature. More accurately, subsistence on tomato seemed to affect more than carrot, which in turns affected more than cucumber. This effect was mostly observed at 4 and 10 \({}^{\circ}\)C. In the latter temperature, _hly_ was downregulated in all cases, while in the former, _hly_ was upregulated in the majority of the cases. Transcription of _hly_ may be initiated by three promoter sites, two of which are PrfA-dependent since they contain a PrfA box [56]. In the present study, activation of the third promoter site is indicated, taking into consideration the moderate correlation between _prfA_ and _hly_ transcription revealed by Pearson correlation coefficient. _plcB_ also holds a very important role in pathogenesis [57]. Subsistence of the pathogen on cucumber affected _plcB_ relative transcription more than tomato and carrot did.