Pyruvate Carboxylase Variants Enabling Improved Lysine Production from Glucose Identified by Biosensor-Based High-Throughput Fluorescence-Activated Cell Sorting Screening

Pyruvate carboxylase is an anaplerotic carbon dioxide-fixing enzyme replenishing the tricarboxylic acid cycle with oxaloacetate during growth on sugars. In this study, we applied a lysine biosensor to identify pyruvate carboxylase variants in Corynebacterium glutamicum that enable improved l-lysine production from glucose. A suitable reporter strain was transformed with a pyc gene library created by error-prone PCR and screened by fluorescence-activated cell sorting (FACS) for cells with increased fluorescence triggered by an elevated cytoplasmic lysine concentration. Two pyruvate carboxylase variants, PCx^T343A,I1012S and PCx^T132A were identified allowing 9% and 19% increased lysine titers upon plasmid-based expression. Chromosomal expression of PCx^T132A and PCx^T343A variants led to 6% and 14% higher l-lysine levels. The new PCx variants can be useful also for other microbial strains producing TCA cycle-derived metabolites. Our approach indicates that a biosensor such as pSenLys enables directed evolution of many enzymes involved in converting a carbon source into the target metabolite.

Recent Advances in Function-based Metagenomic Screening

Metagenomes from uncultured microorganisms are rich resources for novel enzyme genes. The methods used to screen the metagenomic libraries fall into two categories, which are based on sequence or function of the enzymes. The sequence-based approaches rely on the known sequences of the target gene families. In contrast, the function-based approaches do not involve the incorporation of metagenomic sequencing data and, therefore, may lead to the discovery of novel gene sequences with desired functions. In this review, we discuss the function-based screening strategies that have been used in the identification of enzymes from metagenomes. Because of its simplicity, agar plate screening is most commonly used in the identification of novel enzymes with diverse functions. Other screening methods with higher sensitivity are also employed, such as microtiter plate screening. Furthermore, several ultra-high-throughput methods were developed to deal with large metagenomic libraries. Among these are the FACS-based screening, droplet-based screening, and the in vivo reporter-based screening methods. The application of these novel screening strategies has increased the chance for the discovery of novel enzyme genes.