Regulated expression of RecA protein in a bacterial cell. The gene encoding the RecA protein, fused to a bacteriophage T7 promoter, is cloned into an expression vector. Under normal growth conditions (uninduced, left lane), no RecA protein appears when cellular proteins are separated on a polyacrylamide gel (see Chapter 6) and stained with Coomassie Blue for visualization. When the T7 RNA polymerase is induced in the cell (right lane), the recA gene is expressed, and the large amounts of RecA protein produced are readily observed.