The nature of the traK4 mutation in the F sex factor of Escherichia coli.

The sequence of traK gene of the F sex factor of Escherichia coli is presented; the traK gene product is predicted to be a protein of 25,627 Da with a signal sequence of 21 amino acids to give a mature protein of 23,307 Da. The traK4 mutation is an extremely polar mutation in the F plasmid that affects F pilus synthesis and plasmid transfer. traK genes carrying the traK4 mutation and a nonpolar mutation traK105 were cloned, sequenced, and identified as an amber nonsense and a frameshift mutation, respectively. The traK4 mutation occurred within one predicted rho-dependent transcription termination element (TTE) and immediately upstream of another, while the traK105 mutation occurred after the two potential TTEs within the traK gene. S1 nuclease protection analysis and Northern (RNA) blot analysis were used to confirm that the traK4 mutation, but not the traK105 mutation, caused premature termination of transcription. Computer analysis of the F transfer region suggested the presence of TTE motifs at regular intervals throughout the 33.4-kb sequence.