A practical antibiotics-free plasmid expression system in cyanobacteria was developed by using the complementation of cyanobacterial recA null mutation with the Escherichia coli recA gene on the plasmid. This system was applied to the production of polyhydroxyalkanoate (PHA), a biodegradable plastic, and the transgenic cyanobacteria stably maintained the pha genes for PHA production in the antibiotics-free medium, and accumulated up to 52% cell dry weight of PHA. (C) 2011 Elsevier Ltd. All rights reserved.