SUCROSE METABOLISM IN Nitrosomonas europaea

BALLICORA MA; ASENCION DIEZ MD; HARTMAN MD; FIGUEROA CM; FERRETTI MV; IGLESIAS AA

virtual

Congreso; LVII SAIB Meeting - XVI SAMIGE Meeting; 2021

The synthesis of sucrose (Suc) is carried out by the combined action of Suc-6P synthase (SucPSase, EC 2.4.1.14) and Suc-6Pphosphatase (SucPase, EC 3.1.3.24). Nitrosomonas europaea is an ammonium oxidizing bacterium and is classified as achemolithoautotroph organism. N. europaea can grow either autotrophic or heterotrophically when the carbon source is CO2or fructose, respectively. We found that N. europaea has a sequence coding for an ~80 kDa protein highly homologous toSucPSase type II (possessing both SucPSase and SucPase domains). Our previous results showed that the SucPSase type IIdisplayed low SucPSase (0.065 U/mg) and SucPase (0.012 U/mg) activities. Conversely, the SucPSase and SucPase domainsdisplayed activities of 0.33 and 30 U/mg, respectively, when separately expressed. Immunodetection assays against theSucPase domain in crude extracts from N. europaea grown with fructose as sole carbon source showed that the SucPSase typeII protein is expressed in its complete and low-active form. Since the CO2 present in the air can be considered as a limitingsubstrate, we developed a device to grow cells with constant air bubbling using an aeration pump. We observed a notableincrease in OD (2-fold higher) compared to the condition without air supplementation. When we performed immunodetectionon extracts from cells grown under well-aerated, chemolithoautotrophic conditions, we detected the separated, highly-activedomains. Results indicate that the enzyme is present in its low-activity, complete form (~80 kDa) when the bacterium growsheterotrophically, whereas SucPSase and SucPase domains are separated under chemolithoautotrophic conditions. Curiously,we found a gene that codes for a serine peptidase of the S8 family in the genome of N. europaea. This protease would cut theenzyme at the linker of both domains, giving rise to the most active forms of the enzyme. Based on these results, we hypothesizethat sucrose metabolism in N. europaea could be regulated by proteolysis, a fast response to environmental changes.