The eicosapentaenoic acid (EPA) synthesis gene cluster from an EPA-producing bacterium, Shewanella sp. SCRC-2738, was cloned into a broad-host range vector, pJRD215, and then introduced into a marine cyanobacterium, Synechococcus sp. NKBG15041c, by conjugation. The transconjugant cyanobacteria produced 3.7 +/- 0.2% (2.24 +/- 0.13 mg/L) EPA (n-3) and 2.5 +/- 0.2% (1.49 +/- 0.06 mg/L) eicosatetraenoic acid (n-3) of the total fatty acids when the cells were cultured at 23 degrees C at a light intensity of 1,000-1,500 Lux. The EPA and eico-satetraenoic acid contents of the cells were increased to 4.6 +/- 0.6% (3.86 +/- 1.11 mg/L) and 4.7 +/- 0.3% (3.86 +/- 0.82 mg/L), and 7.5 +/- 0.3% (1.76 +/- 0.10 mg/L) and 5.1 +/- 0.2% (1.19 +/- 0.06 mg/L) when they were cultured at low temperature (18 degrees C) and at lower light intensity (40 Lux), respectively.