Physiological characterization and quantitative proteomic analyses of metabolically engineered E. coli K4 strains with improved pathways for capsular polysaccharide biosynthesis

Among capsulated bacteria, some produce polysaccharides with unique properties that have been shown to possess relevant industrial applications and commercial value. The capsular polysaccharide (CPS) produced by Escherichia coli K4 is similar to chondroitin sulphate, and recent efforts focused on the development of genetic and fermentation strategies to increase its production titers up to technologically attractive levels. However, the control of the metabolic pathways leading to CPS synthesis together with the effect of varying the concentration of pathway intermediates on CPS final titers, is still quite unexplored, and not fully understood. In the present study four genes involved in the biosynthesis of UDP-sugar CPS precursors, namely kfoA, kfoF, pgm, and galU, were overexpressed in different combinations, and diversely affected the biosynthetic machinery. At the physiological level, results revealed a central role for kfoF, coding for UDP-glucose dehydrogenase, that increased CPS production mostly. In the attempt to unravel the molecular mechanisms regulating CPS biosynthesis, an in depth analysis of the proteome of the recombinant strains overexpressing respectively pgm and galU, and pgm, galU, and kfoF was performed and compared to the wild-type. Although, interestingly, in both strains the impact of the genetic manipulation seemed rather limited at the proteome level, results obtained from the triple mutant indicated a crosstalk between the two pathways leading to UDP-sugar precursors biosynthesis, and also an unexpected link with the purine biosynthetic pathway. Overall our results present new insights into the role of metabolic intermediates for the formation of capsular polysaccharides, utilizing a systematic approach of metabolic engineering, combined with state-of-the-art quantitative proteomic approaches, as well as genetic and physiological information