Date Published: July 29, 2012
Author(s): Juan A Vallejo, Manuel Serrat, Irasema Pérez-Portuondo, Angeles Sánchez-Pérez, Jose M Ageitos, Tomas G Villa.
Flocculation is a very useful phenotype for industrial yeast strains, since it facilitates cell harvest and represents an easy way of cell immobilization in continuous fermentation processes. The present work represents the first time that an inducible flocculation phenotype has been generated in a non flocculent strain of Kluyveromyces marxianus. This was accomplished by expressing Saccharomyces cerevisiae FLO5 gene in K. marxianus CECT 11769 strain. The FLO 5 gene was placed under the control of an EPG promoter, not repressed by glucose and induced by anoxia. Our experimental approach successfully generated two novel K. marxianus flocculent phenotypes: one inducible and one constitutive. The constitutive phenotype originated from deletions in the FLO5 promoter region, indicating the existence of putative upstream repressor site involved in oxygen regulation of the EPG1 promoter. The novel strains here generated had a unique set of characteristics that provided an advantage, over the wild-type strain, for the industrial co-production of ethanol and polygalacturonase.
Yeast flocculation is defined as an asexual and reversible Ca2+-dependent cellular aggregation that generates fast-sedimenting flocs containing high numbers of yeast cells (Bony et al. ; Stratford, ). Under adverse conditions, the innermost cell layer lyses thus providing nutrients for the cells in the outer layer (Hercker et al. ). Yeast growth under flocculating conditions, while often slow due to the difficulty in the uptake of nutrients, protects the cells from toxic substances accumulating in the media (Van Mulders et al. ). Thus, the floc phenotype confers resistance to ethanol and oxidative stresses (Smukalla et al. ).
Yeasts strains with the ability to flocculate are very useful in industrial applications, as the flocculent phenotype allows for an easier, and cheaper, way of separating the yeast culture from the final, economically-important, fermentation product (Ratledge and Kristiansen, ); and also because of the higher production rate and cellular densities obtained with these strains (Teixeira et al. ). As a result of this, flocculent strains are ideally suited for use in continuous fermentative processes (Domingues et al. [2000b]). However, the flocculent phenotype negatively affects the yeast growth (Zhao et al. ), therefore an inducible flocculent phenotype would be most desirable to use in fermentation procedures (Govender et al. ).
The authors declare that they have no competing interests.