The ethylene-forming enzyme (EFE) from Pseudomonas syringae catalyzes the synthesis of ethylene which can be easily\r\ndetected in the headspace of closed cultures. A synthetic codon-optimized gene encoding N-terminal His-tagged EFE (EFEh)\r\nwas expressed in Synechocystis sp. PCC 6803 (Synechocystis) and Escherichia coli (E. coli) under the control of diverse\r\npromoters in a self-replicating broad host-range plasmid. Ethylene synthesis was stably maintained in both organisms in\r\ncontrast to earlier work in Synechococcus elongatus PCC 7942. The rate of ethylene accumulation was used as a reporter for\r\nprotein expression in order to assess promoter strength and inducibility with the different expression systems. Several\r\nmetal-inducible cyanobacterial promoters did not function in E. coli but were well-regulated in cyanobacteria, albeit at a low\r\nlevel of expression. The E. coli promoter Ptrc resulted in constitutive expression in cyanobacteria regardless of whether IPTG\r\nwas added or not. In contrast, a Lac promoter variant, PA1lacO-1, induced EFE-expression in Synechocystis at a level of\r\nexpression as high as the Trc promoter and allowed a fine level of IPTG-dependent regulation of protein-expression. The\r\nregulation was tight at low cell density and became more relaxed in more dense cultures. A synthetic quorum-sensing\r\npromoter system was also constructed and shown to function well in E. coli, however, only a very low level of EFE-activity\r\nwas observed in Synechocystis, independent of cell density.
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