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Bacterial contaminations in yeast fermentation tanks are a recurring problem for the bioethanol production industry. Lactic acid bacteria (LAB), particularly of the genus Lactobacillus, are the most common contaminants. Their proliferation can reduce fermentation efficiency or even impose premature shutdown for cleaning. We have previously reported that laboratory yeast strains naturally excrete amino acids via transporters of the Drug: H+ Antiporter-1 (DHA1) family. This excretion allows yeast to cross-feed LAB, which are most often unable to grow without an external amino acid supply. Whether industrial yeast strains used in bioethanol production likewise promote LAB proliferation through cross-feeding has not been investigated. In this study, we first show that the yeast strain Ethanol Red used in ethanol production supports growth of Lactobacillus fermentum in an amino-acid-free synthetic medium. This effect was markedly reduced upon homozygous deletion of the QDR3 gene encoding a DHA1-family amino acid exporter. We further show that cultivation of Ethanol Red in a nonsterile sugarcane-molasses-based medium is associated with an increase in lactic acid due to LAB growth. When Ethanol Red lacked the QDR1, QDR2, and QDR3 genes, this lactic acid production was not observed and ethanol production was not significantly reduced. Our results indicate that Ethanol Red cultivated in synthetic or molasses medium sustains LAB proliferation in a manner that depends on its ability to excrete amino acids via Qdr transporters. They further suggest that using mutant industrial yeast derivatives lacking DHA1-family amino acid exporters may be a way to reduce the risk of bacterial contaminations during fermentation.

The rat Chromosome (Chr) 2 harbors several genes controlling tumor growth or development, blood pressure, and non-insulin-dependent diabetes mellitus. We report that the region (2q1) containing the mammary susceptibility cancer gene Mcs1 also harbors the genes encoding cyclin B1, interleukin 6 signal transducer (gp130), and proprotein convertase 1. We also generated 13 new anonymous microsatellite markers from Chr 2-sorted DNA. These markers, as well as a microsatellite marker in the cyclin B1 gene, were genetically mapped in combination with known markers. A cyclin B1-related gene was also cytogenetically assigned to rat Chr 11q22-q23.