The sex factor of colicin factor E1a

Abstract
The sex factor of colE1a appears related to Ib, the sex factor of colIb, by each of three criteria: mutual inhibition of epidemic spread, antigenic structure of the sex pilus and susceptibility to I phage. The failure of each factor to spread in cultures carrying the other implies that donor ability is subject to a cytoplasmic repressor. Unlike two colI factors, colE1a and colIa (or Ib) can co-exist to give a doubly colicinogenic strain.