Engineering Synechococcus elongatus IITB6 as a highly efficient ethanol bioproduction host

Abstract

Cyanobacteria have emerged as attractive hosts for the sustainable photosynthetic conversion of CO₂ to biofuels, especially ethanol. However, the low ethanol titers and productivity achieved so far have limited the industrial translation of the process. Conventional model cyanobacterial host strains exhibit slow growth and large pool of storage molecules which may limit the production of heterogeneous products like ethanol. In this context, we have isolated and characterized a set of fast-growing Synechococcus elongatus strains, of which IITB1(PCC11801) and IITB6 have been promising for metabolic engineering. Here, we engineered the ethanologenic pathway in IITB6 and optimized gene expression levels by screening combinations of native cyanobacterial promoters of varying strength. Expression of pyruvate decarboxylase and NADPH-dependent alcohol dehydrogenase under low-strength promoters coupled with cultivation in 5X concentrated BG-11 medium gave 1.3 g/L ethanol in 4 days, twice that of the previously reported shake-flask titer from pdc-adh-expressed recombinant cyanobacterial strains. This work opens avenues for developing S. elongatus IITB6 as an efficient host for ethanol production.