Metabolic engineering of solventogenic clostridia

Abstract

Metabolic engineering (ME) can be defined as “directed modification of cellular metabolism and properties through the introduction, deletion and/or modification of metabolic pathways using recombinant DNA and other molecular biological techniques,” which is a slight modification of the original definition by Bailey [1]. Recombinant DNA technology has enabled metabolic pathway modification using targeted genetic modifications, in addition to the traditional mutagenesis and selection approach. Since the mid-1980s, numerous examples of metabolic engineering have been reported [2]. It was originally thought that it would be a straightforward task to change the metabolic fluxes in a desired fashion by simply introducing the genes for a new pathway or amplifying or deleting the genes of other metabolic pathways. However, this has not been the case. This is because cells have evolved to use complex schemes to regulate gene expression and enzyme levels in order to economize energy and biosynthetic resources. Thus, modification of cellular traits might require changes in numerous genes that are loosely related to the pathways for a particular cellular trait. For example, it has been recently reported that for the 3-PG Escherichia coli based DuPont/Genencor Process more than 60 genetic modifications proved necessary to generate an industrially significant strain [3]. In addition to maximizing the flux for a desirable product, improved robustness and prolonged productivity of the biocatalyst (the cells) under realistic bioprocessing conditions is an equally important ME goal. Therefore, ME strategies that increase the ability of cells to withstand “stressful” bioprocessing conditions (such as accumulation of toxic products, byproducts, or substrates as encountered in most applications) without loss of productivity, must also be pursued. Identification of key target genes, and implementation of genetic modification make it imperative that advanced highthroughput investigative tools (such as DNA microarrays), as well as crucial genetic tools and strategies (such as easy and precise gene inactivation and screening technologies) are developed and effectively employed.