Screen strategy. (A) Ty3 replication. The S. cerevisiae knockout mutant collection was screened for strains that are increased or decreased for Ty3-HIS3 transposition as described in the Methods. Ty3-HIS3 was expressed by galactose induction from a high-copy, URA3-marked plasmid. Reverse transcription of the Ty3 RNA resulted in a cDNA that transposes by integration or recombination into chromosomal DNA. (B) Genetic assay of deletion mutant collection for transposition. Deletion mutants transformed with the high-copy Ty3-HIS3 plasmid were patched onto SD-Ura, -His medium (SD-UH), and replica plated as shown in the diagram. Papillations on the final selective medium are indicative of colonies derived from cells that have lost plasmids and undergone transposition. Two independent transformants were tested for each mutant. Mutants were rescreened with Ty3-HIS3 expressed from a low-copy plasmid. Up and down indicate patches of mutants with increased and decreased frequencies of transposition. (C) Distribution of Ty3 transposition phenotypes in knockout mutants using high- and low-copy Ty3-HIS3 assays. Mutants were originally screened by using a high-copy Ty3-HIS3 element (left). Mutants that displayed increased (up) and decreased (down) transposition relative to wild type were rescreened by using a low-copy plasmid (right). One hundred thirty mutants displayed consistent phenotypes in high- and low-copy Ty3-HIS3 assays. Up to down, up to wild type, down to up, and down to wild type refer to changes in transposition phenotype between the two vector systems, with the high-copy vector phenotype indicated first.