Gene Organization and Expression in Mesophilic Lactic Acid Bacteria

A detailed understanding of gene structure and expression in lactic acid bacteria is essential for the successful application of genetic modification techniques for both the improvement of these bacteria used in industrial dairy and food fermentations and their use as food-grade production organisms. This paper is intended to review the organization and expression of homologous and heterologous genes in the mesophilic lactic acid bacteria of the genera Lactococcus and Leuconostoc. Specific attention is given to the wellstudied lactococcal lactose and proteinase genes encoding the fermentation of lactose and the degradation of casein, respectively. The detailed molecular analysis of these genes and their gene products has been applied in lactococci to the development of a food-grade marker system and overproduction and complete secretion of a proteinase with an unique caseinolytic specificity. In addition, it enabled the construction of expression and secretion vectors. These have been used for the expression of Escherichia coil lacZ gene fusions encoding ~-galactosidase in Lactococcus and Leuconostoc. Moreover, the functionality of the identified expression and topogenic signals has been demonstrated by the production and secretion of bovine prochymosin in lactococci.