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Type strains of 200 species of yeasts able to ferment glucose and grow on xylose were screened for fermentation of D-xylose. In most of the strains tested, ethanol production was negligible. Nineteen were found to produce between 0.1 and 1.0 g of ethanol per liter. Strains of the following species produce more than 1 g of ethanol per liter in the fermentation test with 2% xylose: Brettanomyces naardenensis, Candida shehatae, Candida tenuis, Pachysolen tannophilus, Pichia segobiensis, and Pichia stipitis. Subsequent screening of these yeasts for their capacity to ferment D-cellobiose revealed that only Candida tenuis CBS 4435 was a good fermenter of both xylose and cellobiose under the test conditions used.

Linear DNA plasmids were found in the following yeasts: four strains of Kluyveromyces lactis, one of Debaryomyces hansenii, one of Wingea robettsiae and four of Pichia etchellsii. In each case, the plasmids were present as a pair of DNA molecules of different sizes. The plasmids of K. lactis strains were associated with a killer activity and their structure was similar to the known killer plasmids pGKLl and 2. The plasmids from the other three species were different from pGKL plasmids and showed no killer activity against the yeast species tested so far. In all cases, the linear molecules possessed terminal (probably inverted) repeats and their 5' ends had a protected structure insensitive to 1 exonuclease, while the 3' ends were accessible to exonuclease 111. All these strains could be efficiently cured of the plasmids by ultraviolet irradiation. The plasmids from D. hansenii (pDH1A and B) and from W. robettsiae (pWR1A and B) shared related sequences with some of the K. lactis killer plasmid genes (encoding the supposed DNA polymerases, RNA polymerase and the chitinase), suggesting related genome organization of these plasmids. The pair of plasmids from P. etchellsii (pPE1A and B) appear to be a distantly related member of the group. This pair showed no sequence homology with other plasmids, except weak homology with the putative RNA polymerase gene of pGKL2. None of the plasmids contained the sequences homologous to ORF3 and ORF4 of pGKLl encoding the toxin resistance determinant and the toxin y subunit, respectively.