Screening for conditions of enhanced production of a recombinant β-glucanase secreted into the medium by Escherichia coli

Secretion of recombinant proteins from E. coli

The microorganism Escherichia coli is commonly used for recombinant protein production. Despite several advantageous characteristics like fast growth and high protein yields, its inability to easily secrete recombinant proteins into the extracellular medium remains a drawback for industrial production processes. To overcome this limitation, a multitude of approaches to enhance the extracellular yield and the secretion efficiency of recombinant proteins have been developed in recent years. Here, a comprehensive overview of secretion mechanisms for recombinant proteins from E. coli is given and divided into three main sections. First, the structure of the E. coli cell envelope and the known natural secretion systems are described. Second, the use and optimization of different one- or two-step secretion systems for recombinant protein production, as well as further permeabilization methods are discussed. Finally, the often-overlooked role of cell lysis in secretion studies and its analysis are addressed. So far, effective approaches for increasing the extracellular protein concentration to more than 10 g/L and almost 100% secretion efficiency exist, however, the large range of optimization methods and their combinations suggests that the potential for secretory protein production from E. coli has not yet been fully realized.

Extracellular recombinant protein production under continuous culture conditions with Escherichia coli using an alternative plasmid selection mechanism

The secretion of recombinant proteins into the extracellular space by Escherichia coli presents advantages like easier purification and protection from proteolytic degradation. The controlled co-expression of a bacteriocin release protein aids in moving periplasmic proteins through the outer membrane. Since such systems have rarely been applied in continuous culture it seemed to be attractive to study the interplay between growth-phase regulated promoters controlling release protein genes and the productivity of a chemostat process. To avoid the use of antibiotics and render this process more sustainable, alternative plasmid selection mechanisms were required. In the current study, the strain E. coli JM109 harboring plasmid p582 was shown to stably express and secrete recombinant β-glucanase in continuous culture using a minimal medium. The segregational instability of the plasmid in the absence of antibiotic selection pressure was demonstrated. The leuB gene, crucial in the leucine biosynthetic pathway, was cloned onto plasmid p582 and the new construct transformed into an E. coli Keio (ΔleuB) knockout strain. The ability of the construct to complement the leucine auxotrophy was initially tested in shake-flasks and batch cultivation. Later, this strain was successfully grown for more than 200 h in a chemostat and was found to be able to express the recombinant protein. Significantly, it showed a stable maintenance of the recombinant plasmid in the absence of any antibiotics. The plasmid stability in a continuously cultivated E. coli fermentation, in the absence of antibiotics, with extracellular secretion of recombinant protein provides an interesting model for further improvements.

Production of phloroglucinol by Escherichia coli using a stationary-phase promoter

Escherichia coli was metabolically engineered using a new host-vector system to produce phloroglucinol. The key biosynthetic gene phlD (encoding a type III polyketide synthase) from Pseudomonas fluorescens was expressed in E. coli using the stationary-phase promoter of the fic gene and a high-copy plasmid. In shake-flasks, the engineered strain produced phloroglucinol up to 0.28 g/l with a productivity of 0.014 g/l h. About 9.2% of the glucose consumed was converted to phloroglucinol after 20 h. Compared with the widely used inducible T7 promoter system, this strain did not require IPTG induction and the final titer of phloroglucinol was 22% higher.