Use of second-site suppressor mutations in Drosophila to identify components of the transcriptional machinery.

Isolation of second-site suppressor mutations provides a powerful method for identifying (i) genes that encode proteins that interact and (ii) domains within the interacting proteins that contact each other. Flies conditionally lethal because they carry mutations in the largest subunit of RNA polymerase II were mutagenized; ten million progeny were then screened for compensatory mutations. Eight intragenic and 10 extragenic suppressor mutations were recovered. Both the conditional lethality and premature termination of transcription caused by one mutation in the largest subunit of RNA polymerase II are compensated by an allele-specific suppressor mutation in the second-largest subunit of the enzyme.