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The natural habitat of Kluyveromyces lactis is diverse, but many strains were originally isolated from milk-derived products. At present, two varieties in K. lactis have been defined by taxonomists: K. lactis (Dombrowski) van der Walt var. lactis (1986) and K. lactis (Dombrowski) van der Walt var. drosophilarum (1986) . The former can assimilate lactose while the latter does not.
K. lactis is a scientifically and biotechnologically non-Saccharomyces important yeast. Although it is an ascomycetous budding yeast closely related to S. cerevisiae, K. lactis has become an attractive alternative model owing to distinct metabolic and physiological properties. It is a Crabtree-negative aerobic respiro-fermentative yeast, which can accommodate a wide range of carbon sources such as lactose and other respiratory substrate . Biogenesis of its respiratory system is not sensitive to glucose repression but K. lactis also ferments glucose to ethanol in aerobic conditions. Industrial interests also stimulate research on K. lactis since it is able to grow to a high cell density and to secrete heterologous proteins efficiently.

K. lactis is usually grown in standard yeast medium at an optimal temperature of 28°C. Its life cycle is essentially similar to that of S. cerevisiae, except that diploids are not stable and sporulate rapidly. K. lactis is amenable to genetic studies using conventional techniques such as crossing and tetrads analysis, gene targeting, integrative and replicative transformation.
Electrophoretic karyotyping and genome sequencing distinguishes six chromosomes ranging in size from about 1 to 3 Mb . The genome size is 10.6 Mb long (without the rDNA) with approximately 5,300 CDS and 162 tRNA genes. Mitochondrial DNA is a circular molecule with a length of about 40 kb . Some strains contain a pair of cytoplasmic linear DNA plasmids, pGKL1 (8.8 kb) and pGKL2 (13.4 kb), conferring the killer phenotype . A 2µ-like plasmid, pKD1 (4757 bp) has been isolated in some K. lactis strains and a variety of plasmid vector systems, including low/high shuttle plasmids, are also available.

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