Regulation of the conserved hypothetical operon sdhEygfX and its control of cellular processes

Serratia sp. ATCC 39006 (Serratia 39006) is a Gram-negative enterobacterium that produces a number of secondary metabolites including the red-pigmented antibiotic prodigiosin (pig). Although the physiological role of pig remains unknown, it is of considerable clinical interest due to its immunosuppressive and anti-cancer properties. Furthermore, regulation of pig biosynthesis is complex, and involves many proteins that respond to various physiological and environmental cues. Conserved hypothetical proteins are proteins of unknown function present in multiple species, and are hypothesised to represent crucial, yet undiscovered elements of important biological pathways. Two conserved hypothetical proteins, termed SdhE and YgfX, were previously identified as positive regulators of pig biosynthesis, and were hypothesised to form a non- canonical two-component system present in multiple enteric bacteria. Here, YgfX is proposed to function as an inner membrane bound signal sensor that relays an unknown input signal(s) to the hypothesised output protein SdhE. At the beginning of this research project, very little was known about how the SdhE-YgfX system is regulated or how it elicits its cellular effects. This project has identified numerous potential regulators of sdhEygfX expression, of which the protein products of rsmC, rsmA and rseA have been validated as negative regulators. Additionally, several yet uncharacterised regulators of pig biosynthesis have been uncovered, some of which may be involved in the cellular function of SdhE and YgfX. The findings presented in this thesis suggest SdhE-YgfX links cellular metabolism, envelope stress and motility in Serratia 39006