Selection and analysis of rare second-site suppressors of Drosophila RNA polymerase II mutations.

We used a mutagenesis and selection procedure in Drosophila melanogaster to recover rare allele-specific suppressor mutations. More than 11 million flies mutant for one of five recessive-lethal mutations in the two largest subunits of RNA polymerase II were selected for additional mutations that restored viability. Forty-one suppressor mutations were recovered. At least 16 are extragenic, identifying a minimum of three loci, two of which do not map near genes known to encode subunits of RNA polymerase II. At most, 25 are intragenic, 4 reverting the initial altered nucleotide back to wild type. Sequence analysis of interacting mutations in the two largest subunits identified a discrete domain in each subunit. These domains might be contact points for the subunits. Finally, our selections were large enough to allow recovery of multiple independent changes in the same nucleotides yet mutations in other equally likely targets were not recovered. The mutations recovered are not random and might provide insights into possible mechanisms for mutagenesis in eukaryotes.