Escherichia coli K-12 possesses a large number of chromosomal genes that, in other Gram-negative bacteria, are involved either in exoprotein secretion or in the formation of type IV pili. Some of these E. coli genes have been shown to encode proteins when expressed from heterologous promoters. Furthermore, at least two of these proteins are functional in heterologous complementation tests, but none of the genes examined so far is expressed when E. coli is grown under standard laboratory conditions. We propose that transcription of these genes is turned off during growth in laboratory medium, that their expression is controlled by environmental sensor proteins and that they could play an important role in pathogenicity or in the utilization of large polymers.