Cell--cell interactions in conjugating Escherichia coli: role of traT protein in surface exclusion

Escherichia coli cells carrying the F sex factor are poor recipients in conjugation. This phenomenon is called surface exclusion. Two F cistrons, traS and traT, are independently responsible for part of the whole mechanism. The traS gene product reduces DNA transfer within stable mating aggregates. The traT gene product, pTraT, results in a greatly reduced ability to form stable mating aggregates, and thus also leads to reduced DNA transfer within the cell population. pTraT is a 25,000-dalton protein incorporated into the cell envelope outer membrane. It is found in 29,000-84,000 copies per cell, depending on the plasmid expressing it. There is a parallel variation in recipient ability. Models for surface exclusion are discussed.