SECRETERS - A new generation of microbial expression hosts and tools for the production of biotherapeutics and high-value enzymes (Horizon 2020 EU-funded project)

The SECRETERS European research training network aims to develop powerful new microbial platforms for the production of secreted high-quality recombinant proteins, with a special emphasis on therapeutic proteins (biotherapeutics) and industrial enzymes that are difficult to express. Both are important products for the EU biotechnology sector, with combined markets of over $140 billion per year. However, many proteins in these categories, especially disulphide-bonded proteins, pose severe problems in production.

The regulon controlled by the alternative sigma factor SigB of B. subtilis comprises more than 200 genes, and activation of SigB leads to massive changes in the gene expression pattern of B. subtilis. Although the general stress response of B. subtilis is already very well characterised, there are several aspects that are not yet fully understood and are the subject of our research in this area. These include, in particular, the regulatory mechanisms responsible for specific expression patterns of subsets of the SigB regulon and the functional elucidation of SigB-regulated genes. An important focus of our work is also the integration of the SigB-dependent response into the regulatory network of non-growing cells.

B. subtilis is found especially in the upper soil layers, where it is exposed to constant changes in the osmolarity of its environment. Due to desiccation of the soil on the one hand and heavy rainfall on the other, the osmotic conditions in the soil can fluctuate very strongly. To counteract dehydration and plasmolysis of the cells when external osmolarity increases, B. subtilis uses osmo-protective substances, so-called compatible solutes, such as proline and glycine betaine. These are accumulated intracellularly in high concentrations, enabling the cells to retain water and maintain turgor. In close cooperation with the research group of Prof. Erhard Bremer (Marburg), we analyse the regulation of synthesis and uptake systems for osmoprotective substances as well as the global changes in gene expression and physiology of B. subtilis under hyperosmotic conditions.

We are investigating the adaptation to stress and nutrient conditions to which S. aureus is exposed in its interaction with the host, mainly using in vivo proteomic analyses (topic Host-pathogen interactions). A major goal of comparative analyses of wild-type S. aureus with strains carrying mutations in key regulators is to gain a deeper understanding of the regulatory networks that control the adaptation of bacterial physiology to the host environment and the synthesis of virulence factors. The alternative sigma factor SigB is a central regulator that also controls a large regulon in S. aureus, whose genes encode proteins that are involved in stress responses, cell wall and metabolic functions, among other things, but are also virulence factors. Our research group is particularly concerned with the importance of SigB for the survival of S. aureus in eukaryotic cells.

Another focus is the characterisation of the physiological role of the transcription termination factor Rho. As in other bacteria, an essential function of Rho in S. aureus is the repression of antisense transcription. Of particular interest, however, is that in S. aureus there is a pathophysiologically relevant link between the activity of Rho and the regulation of virulence factor genes, the molecular basis of which is being investigated in current work. In addition, we are characterising non-coding RNAs, which are also essential components of regulatory networks.