Genetic dissection of the transcription cycle. A mutant RNA polymerase that cannot hold onto a promoter

Deletion of 10 amino acids from a conserved motif in the beta subunit of Escherichia coli RNA polymerase (RNAP) leads to an interrupted transcription cycle and lethal phenotype. RNAP carrying the mutant subunit retains catalytic function and specificity of promoter recognition but is unable to efficiently hold onto DNA in the binary complex, resulting in a diminished initiation frequency. However, inefficient initiation by the mutant enzyme leads to processive and stable ternary elongating complex. Thus, the mutation dissects the traits of promoter selectivity, binary complex stability, and ternary complex processivity reflecting compartmentalization of function within the RNAP molecule.