Physiological effects of overexpressed sigma factors on fermentative stress response of Zymomonas mobilis

Abstract

Zymomonas mobilis is a bacterium of industrial interest due to its high ethanol productivity and high tolerance to stresses. Although the physiological parameters of fermentation are well characterized, there are few studies on the molecular mechanisms that regulate the response to fermentative stress. Z. mobilis ZM4 presents five different sigma factors identified in the genome annotation, but the absence of sigma 38 leads to the questioning of which sigma factors are responsible for its mechanism of fermentative stress response. Thus, in this study, factors sigma 32 and sigma 24, traditionally related to heat shock, were tested for their influence on the response to osmotic and ethanol stress. The overexpression of these sigma factors in Z. mobilis ZM4 strain confirmed that both are associated with heat shock response, as described in other bacteria. Moreover, sigma 32 has also a role in the adaptation to osmotic stress, increasing both growth rate and glucose influx rate. The same strain that overexpresses sigma 32 also showed a decrease in ethanol tolerance, suggesting an antagonism between these two mechanisms. It was not possible to conclude if sigma 24 really affects ethanol tolerance in Z. mobilis, but the overexpression of this sigma factor led to a decrease in ethanol productivity.

Impact of hfq and sigE on the tolerance of Zymomonas mobilis ZM4 to furfural and acetic acid stresses

“…They revealed that the expression level of sigE enhanced by 8.2 and 6-fold at pH 3.8 and in 12% ethanol, respectively. Our findings on furfural, acetic acid and multiple inhibitor compounds in sugarcane bagasse hydrolysate, along with Zhang et al (2010) outcomes on low pH and ethanol stress and Benoliel et al (2019) on high temperature, confirmed Seo's theory (2005) about the critical role of sigE in the response to various stress conditions[15,20,35].…”

“…Benoliel et al (2019) overexpressed sigE in Z. mobilis and evaluated the recombinant strain in response to high temperatures and ethanol stresses. According to their results, the overexpression of sigE led to significant improvement in high temperature tolerance, but there is no clear evidence that this sigma factor protects Z. mobilis in ethanol stress [15].…”

“…Recently, Benoliel et al (2019) overexpressed sigE in Z. mobilis and their qPCR results showed 169-fold enhanced expression compared to the control. They reported that this high amount of SigE protein is significantly associated with the improvement in tolerance to high-temperature conditions but there is no clear evidence that this protein protects Z. mobilis in ethanol stress [15].…”

“…These findings, together with the transcriptome analysis, have confirmed the critical role of regulatory and sigma factors in Z. mobilis cellular responses to various stresses. In this context, many efforts are focused on engineering the transcriptional regulators and transcription machinery, small RNAs and RNA chaperones [14][15][16][17][18].…”

“…Seo et al proposed that rpoE (sigma E) in Z. mobilis is a RpoS-like regulatory factor and plays a critical role in various stress conditions [20]. Recently, Benoliel et al (2019) overexpressed sigma 32 and sigma E in Z. mobilis and confirmed that both of them are related to heat shock stress [15].…”