Constitutive expression of Vitreoscilla haemoglobin in Sphingomonas elodea to improve gellan gum production.

Aims:  To improve a commercially used strain for gellan production by exogenous Vitreoscilla haemoglobin (VHb). Methods and Results:  VHb gene was expressed in Sphingomonas elodea under the control of constitutive bla promoter. Biochemical activity of expressed VHb was confirmed by CO-difference spectra analysis that exhibited a characteristic absorption maximum at 419 nm. During cultivation, not only enhanced cell growth was detected, but also 20% improvement in gellan production was observed after 48 h of incubation, with a maximum yield of 16·82 g l−1. Moreover, maximum sucrose conversion efficiency (g gellan per g sucrose) was 57·8, 20% higher than that of the parental strain. We further examined the polysaccharide production of VHb-expressing strain at different aeration levels in Erlenmeyer flasks. Again, in all cases, a significant enhancement of gellan production was observed, and the enhancement was more significant under oxygen-limiting conditions (up to 26·8%). Conclusions:  VHb exhibited positive effect on cell growth and gellan yield of S. elodea, especially under hypoxic conditions. Significance and Impact of the Study:  This is the first application of VHb as an effective metabolic engineering strategy in S. elodea to regulate cell growth and optimize gellan yield.