FIG. 1.

Nucleotides and amino acids added following transposon insertion and subsequent removal with PmeI. Three codons of the target sequence are shown, with numerals representing specific bases. x₁, x₂, and x₃ represent the amino acids encoded by codons 123, 456, and 789, respectively. Insertion occurs in one of three reading frames: frame 1 (following base 5), frame 2 (following base 6), or frame 3 (following base 7). The 5-base target site duplication that results from the transposon insertion is underlined, and the amino acids added relative to the original sequence are boxed in gray. Some added amino acids are invariant, whereas others (represented by α, β, γ, and δ) are variable and dependent on the target sequence. Insertions in frame 1 almost always result in premature truncation of the expressed protein. However, in the rare case when codon 456 is itself a stop codon, an insertion in frame 1 will actually add two residues (ɛV, where ɛ = Y or C) to the C terminus of the expressed protein rather than truncate prematurely. α = any amino acid except M, E, K, Q, or W (in approximately 75% of cases it will be x₂); β = I, K, M, N, R, S, or T; γ = L, M, or V; δ = H or Q.