Activation of colanic acid biosynthesis linked to heterologous expression of the polyhydroxybutyrate pathway in Escherichia coli

The biosynthesis of colanic acid (CA) in Escherichia coli was known to be activated during growth at low temperature using sub-optimal medium. However, in this study, an E. coli transformant S173-H (S17-3 with plasmid pBhya-CAB) was found to be able to excrete high amount of CA (10.39 g/L) in glucose supplemented Luria-Bertani medium (LBG) when growing at 37 °C. Inoculation of cells in low pH medium was required for the derepression of the CA regulon, another indispensable requirement was the use of high copy number plasmid for over-expression of the heterologous polyhydroxybutyrate (PHB) biosynthesis pathway in S17-3. In addition, S173-H exhibited superior growth performance in LBG, the maximal cell density (OD600) of cultures reached 40.0, far exceeding that of any known E. coli strains cultivated under similar conditions. Genomic data mining and transcriptional analysis hinted that the persistent growth or CA production might be modulated by interplaying regulation networks that signal the level of messenger substrate, acetyl-CoA or acetylphosphate. Depletion of these messenger substrates may be triggered by efficient PHB biosynthesis that links to enhanced capability in NADPH regeneration in S17-3, due to mutations on loci at pgi, csrA, or other sites.