Due to the need for improving fuel ethanol production processes especially when lignocellulosic biomass is used, the development of microorganism strains with better performance in terms of ethanol yield and productivity is required, par­ticularly concerning the direct conversion of polymeric feedstocks. For instance, a way to improve the technoeconomic indexes of ethanol production processes from starch consists in developing yeasts able to hydrolyze this polysaccharide and then ferment the glucose formed without the addition of amylases. Moreover, the effective utilization of alternative feedstocks to obtain ethanol as the lignocel — lulosic residue requires microorganisms with traits difficult to find in a single spe­cies (cellulase production, pentose assimilation, high ethanol yields, high ethanol tolerance, among others). The native strains of microorganisms cannot meet all these exigencies; therefore, their genetic modification is needed.

Of the thousands of genes contained in the microbial DNA, 90 to 95% are repressed, i. e., at a given moment, the microbial cells require only the expression of a reduced amount of genes for them to accomplish all their metabolic func­tions directed to cell biomass growth. This implies that most genes are involved in complex regulation processes so that the information contained in them sup­plies the necessary instructions for the cells to produce myriad metabolites. From an industrial viewpoint, the higher the number of genes expressed, the wider the spectrum of potential value-added products synthesized as well as the greater the number of substrates that can be assimilated by the microorganism. This is accomplished through the synthesis of a higher number of enzymes responsible for that wider assimilation. Similarly, if the natural repression of the genes encod­ing the production of a given metabolite is eliminated, significant increases in the yield of that metabolite can be reached, though the cell growth may be reduced. In this way, super-producing strains of microorganisms can be developed for pro­duction of different products with commercial importance. Therefore, the selec­tion of an industrial strain of microorganisms requires a selection program that can include the genetic modification of native strains of those microorganisms. The genetic modification of industrial microorganisms can mainly be done in two ways: random modification of DNA and directed modification.