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Local adaptations within species are often governed by several interacting genes scattered throughout the genome. Single-locus models of selection cannot explain the maintenance of such complex variation because recombination separates co-adapted alleles. Here we report a previously unrecognized type of intraspecific multi-locus genetic variation that has been maintained over a vast period. The galactose (GAL) utilization gene network of Saccharomyces kudriavzevii, a relative of brewer's yeast, exists in two distinct states: a functional gene network in Portuguese strains and, in Japanese strains, a non-functional gene network of allelic pseudogenes. Genome sequencing of all available S. kudriavzevii strains revealed that none of the functional GAL genes were acquired from other species. Rather, these polymorphisms have been maintained for nearly the entire history of the species, despite more recent gene flow genome-wide. Experimental evidence suggests that inactivation of the GAL3 and GAL80 regulatory genes facilitated the origin and long-term maintenance of the two gene network states. This striking example of a balanced unlinked gene network polymorphism introduces a remarkable type of intraspecific variation that may be widespread., Genetic variation fuels evolution by providing the diversity on which natural selection acts. In contrast with the more common directional and stabilizing forms of selection' balancing selection favours the maintenance [...]

Monoubiquitination of histone H2B, catalyzed by Rad6-Bre1, is required for methylation of histone H3 on lysines 4 and 79, catalyzed by the Set1-containing complex COMPASS and Dot1p, respectively. The Paf1 protein complex, which associates with RNA polymerase II, is known to be required for these histone H3 methylation events. During the early elongation stage of transcription, the Paf1 complex is required for association of COMPASS with RNA polymerase II, but the role the Paf1 complex plays at the promoter has not been clear. We present evidence that the Paf1 complex is required for monoubiquitination of histone H2B at promoters. Strains deleted for several components of the Pall complex are defective in monoubiquitination of histone H2B, which results in the loss of methylation of lysines 4 and 79 of histone H3. We also show that Paf1 complex is required for the interaction of Rad6 and COMPASS with RNA polymerase II. Finally, we show that the Paf1 complex is required for Rad6-Bre1 catalytic activity but not for the recruitment of Rad6-Bre1 to promoters. Thus, in addition to its role during the elongation phase of transcription, the Paf1 complex appears to activate the function but not the placement of the Rad6-Bre1 ubiquitinprotein ligase at the promoters of active genes. [ABSTRACT FROM AUTHOR]