# The Biology of _Pichia membranifaciens_ Killer Toxins
## 1 Introduction  
Killer yeast strains have the characteristic of secreting toxins of proteinaceous nature that are lethal to sensitive yeast cells and filamentous fungi. The killer phenomenon in yeasts was first discovered in _Saccharomyces cerevisiae_[1] but soon shown to be present in many other genera of yeast [2; 3; 4]. Until now, the killer phenomenon has been discovered in almost one hundred species of more than twenty genera [5]. Over \(11\) different killer toxins have been described, and they are produced by representatives of such genera as _Hanseniispora_, _Pichia_, _Saccharomyces cerevisiae_, _Toruslpora_, _Ustilago_, _Williopsis_, etc. (Table 1) [6; 7; 8; 9; 10]. Killer yeasts are widespread in nature where they can be found in percentages that several fold exceed those found in laboratory strains, indicating the existence of a competitive advantage. Microbes have evolved to compete over the neighborhoods of their same niche. One of the mechanisms to gain advantage that have been described is shown by some killer yeast through the presence of dsRNA viruses. These mycoviruses encode killer toxins that provide benefits to the producing cells by killing sensitive yeast cells. Killer toxin production has been related in _S. cerevisiae_ with the presence of two dsRNA viruses: L-A, the helper virus, and the M killer virus that encodes a killer toxin that determines its phenotype (K1, K2, K28 or Klus). Although killer toxins could imply an important competitive advantage, it has also been demonstrated that there is a fitness cost for carrying mycoviruses [11]. In addition to the cytoplasmic dsRNA viruses mentioned above, a variety of yeast species (_Kluyveromyces lactis, P. acaciae, P. inositolova, D. robertsiae, Trichosporon pullulans_, etc.) produce linear dsDNA plasmids, probably not encapsulated, that encoded killer toxins. Furthermore, chromosomally encoded killer toxins (_Williopsis saturnus, Pichia anomala, Pichia kluyveri, Pichia membraninaraciens_, etc.) are also found widespread in nature. Most of these toxins exceed the focus of the presented review but their characteristics and potential applications can be reviewed in the tables showed (Tables 1-3). The reasons why killer yeasts are immune to their own toxins remain to be elucidated for most killer systems. However, in particular cases it has been solved. For example, K28 immunity occurs via the ubiquitination of re-internalized mature toxin and unprocessed precursor moieties complexes that are degraded by the proteasome [12]. Furthermore, _Pichia acaciae, Kluyveromyces lactis_ and _Debaryomyces robertsiae_ cytoplasmic virus like elements encode toxic anticodon nucleases along with specific proteins that confer toxin immunity [13; 14]. The potential use of killer yeasts and their toxins has been intended for various fields of application such as the alcohol fermentation industries (brewery, winery, and distillery), fermented vegetables, biological control of post-harvest diseases, yeast bio-typing, as antimycotics in the medical field and they have been used as model systems to understand eukaryotic polypeptide processing and expression of eukaryotic viruses [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Currently, around 1500 different species of yeasts are known [22] and one of the largest yeast genera is the genus _Pichia_ within the context of number of species. The number of species included in _Pichia_ genus has changed considerably since [PERSON] (1904) first described the genus _Pichia_[22; 23; 24; 25]. _Pichia_ species are widely distributed and are present in natural habitats and also as spoilage yeasts in several foods and beverages [26; 27] and even, human pathogens [28; 29]. Killer phenomenon has been discovered and studied in several species of _Pichia_ that show killer activity against yeasts or fungi of clinical concern (Table 2). Toxins of the genus _Pichia_ are either associated with cytoplasmic genetic elements, such as dsDNA virus-like elements or chromosomally encoded [30; 31; 32].