Cyanobacteria are the earliest prokaryotes that perform oxygenic photosynthesis on the earth. Synechocystis sp. PCC6803, a unicellular cyanobacterium, is able to utilize light energy to grow autotrophically or glucose to grow heterotrophically. It is one of the model species used in the molecular genetics study of cyanobacteria. Biomembranes of Synechocystis 6803 include outer membranes, plasma membranes and thylakoid membranes. The synthesis of membrane lipids is initiated by two acylation reactions of G3P (sn-glycerol-3-phosphate). In prokaryotes, the acylation at sn-1 position is performed through PlsB or the PlsX/Y pathway. The first pathway is catalyzed by PlsB (GPAT, sn-glycerol-3-phosphate acyltransferase), the second pathway is coordinately catalyzed by PlsX (phosphate transferase) and PlsY (acyltransferse). The acylation at sn-2 position is catalyzed by PlsC (LPAAT, 1-acyl-glycerol-3-phosphate acyltransferase). The target of this study is to identify genes involved in the synthesis of membrane lipids in Synechocystis sp. PCC 6803 and regulate the expression of the genes. Approaches and results are summarized as follows:
1. By complementing E. coli BB13 (plsB-plsX -), a mutant defective in synthesis of membrane lipids, with a gene library of Synechocystis sp. PCC 6803, we identified slr1510 as an important gene involved in the synthesis of membrane lipids. slr1510 is similar to plsX and predicted to encode the phosphate transferase involved in the acylation at sn-1 position of G3P. Western blot detection indicated that PlsX is located in the total membranes of the cyanobacterium.
2. A copper-regulated promoter PpetE was used to regulate the expression of slr1510. When the expression of slr1510 was tuned off, the cyanobacterium no longer showed growth. In addition, pigments were significantly reduced, thylakoid membranes were destructed, oxygenic photosynthesis and respiration were greatly lowered or ceased and proteins were degraded. All these results supported that slr1510 played a key role in the synthesis of membrane lipids.