Nitric oxide synthases from photosynthetic organisms improve growth and confer nitrosative stress tolerance in E. coli. Insights on the pterin cofactor

Abstract

Nitric oxide synthase (NOS) synthesizes NO from the substrate L-arginine (Arg). NOS with distinct biochemical properties were characterized from two photosynthetic microorganisms, the unicellular algae Ostreococcus tauri (OtNOS) and the cyanobacteria Synechococcus PCC 7335 (SyNOS). In this work we studied OtNOS and SyNOS recombinantly expressed in E. coli and analyzed bacterial growth and tolerance to nitrosative stress. Results show that the expression of OtNOS and SyNOS promotes bacterial growth and allows metabolizing Arg as N source. In accordance to a high NO producing activity, OtNOS expression induces the hmp flavohemoglobin in E. coli, suggesting that this strain is sensing nitrosative stress. The addition of 1 mM of the NO donor sodium nitroprusside (SNP) is toxic and generates a strong nitrosative stress. The expression of OtNOS or SyNOS reduced SNP toxicity restoring bacterial growth. Finally, using bioinformatic tools and ligand docking analyses, we propose tetrahydromonapterin (MH4), an endogenous pterin found in E. coli, as potential cofactor required for NOS catalytic activity. Our findings could be useful for the development of biotechnological applications using NOS expression to improve growth in bacteria.