Effects of global transcription factor NtcA on photosynthetic production of ethylene in recombinant Synechocystis sp. PCC 6803

Huilin Mo, Xiaoman Xie, Tao Zhu, Xuefeng Lu

Cyanobacteria are considered potential photosynthetic microbial cell factories for biofuel and biochemical production. Ethylene, one of the most important organic chemicals, has been successfully synthesized in cyanobacteria by introducing an exogenous ethylene-forming enzyme (Efe). However, it remains challenging to significantly improve the biosynthetic efficiency of cyanobacterial ethylene. Genetic modification of transcription factors is a powerful strategy for reprogramming cellular metabolism toward target products. In cyanobacteria, nitrogen control A (NtcA), an important global transcription regulator of primary carbon/nitrogen metabolism, is expected to play a crucial role in ethylene biosynthesis. The partial deletion of ntcA (MH021) enhanced ethylene production by 23%, while ntcA overexpression (MH023) in a single-copy efe recombinant Synechocystis (XX76) reduced ethylene production by 26%. Compared to XX76, the Efe protein content increased 1.5-fold in MH021. This result may be due to the release of the negative regulation of NtcA on promoter P cpcB , which controls efe expression. Glycogen content showed a 23% reduction in MH021, and the ratio of intracellular succinate to 2-oxoglutarate (2-OG) increased 4.8-fold. In a four-copy efe recombinant strain with partially deleted ntcA and a modified tricarboxylic acid (TCA) cycle (MH043), a peak specific ethylene production rate of 2463 ± 219 μL L(-1) h(-1) OD730(-1) was achieved, which is higher than previously reported. The effects of global transcription factor NtcA on ethylene synthesis in genetically engineered Synechocystis sp. PCC 6803 were evaluated, and the partial deletion of ntcA enhanced ethylene production in both single-copy and multi-copy efe recombinant Synechocystis strains. Increased Efe expression, accelerated TCA cycling, and redirected carbon flux from glycogen probably account for this improvement. The results show great potential for improving ethylene synthetic efficiency in cyanobacteria by modulating global regulation factors.