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Leuconostoc lactis  AV1 strain isolated from a Tunisian avocado was characterized as a dextran producer. The promoter P  dsrLL   and the  dsrLL  gene encoding the DsrLL dextransucrase responsible for the dextran synthesis were transcriptionally fused to the mCherry coding gene generating the pRCR20 plasmid. Upon plasmid transfer, both AV1n and the dextran non-producing  Leuconostoc mesenteroides  CM70 became red due to expression of the mCherry from the P  dsrLL -dsr-mrfp  transcriptional fusion. Characterization of the polymers present in cultures supernatants revealed that the DsrLL encoded from pRCR20 in the recombinant bacteria was able to synthesize dextran. The production of dextran by the DsrLL in AV1n increased in response to low temperature, reaching 10-fold higher levels at 20°C than at 37°C (4.15 g/L versus 0.41 g/L). To analyze if this stress response includes activation at the transcriptional level and if it was only restricted to  Leuconostoc , AV1n was transformed with plasmids carrying either the P  dsrLL  - mrfp  fusion or the P  dsrLS   of  Lactobacillus sakei  MN1 fused to the  mrfp  gene, and the influence of temperature and carbon source on expression from the Dsr promoters was monitored by measurement of the mCherry levels. The overall expression analysis confirmed an induction of expression from P  dsrLL   upon growth at low temperature (20°C versus 30°C and 37°C) in the presence of sugars tested (sucrose, glucose, maltose, and fructose). In addition, the presence of sucrose, the substrate of Dsr, also resulted in activation of expression from P  dsrLL  . A different behavior was detected, when expression from P  dsrLS   was evaluated. Similar levels of fluorescence were observed irrespectively of the carbon source or temperature, besides a sequential decrease at 30°C and 20°C, when sucrose was present in the growth medium. In conclusion, the two types of regulation of expression of Dsr presented here revealed two different mechanisms for environmental adaptation of  Leuconostoc  and  Lactobacillus  that could be exploited for industrial applications.

Different Modes of Regulation of the Expression of Dextransucrase in Leuconostoc lactis AV1n and Lactobacillus sakei MN1