Abstract
Microcins are ribosomally synthesized peptide antibiotics that are produced by enterobacterial strains. Although the first studies concentrated on plasmid-encoded activities, in the last years three chromosome-encoded microcins have been described: H47, E492, and M. Here, a new microcin, I47, is presented as a fourth member of this group. Common features exhibited by chromosome-encoded microcins were searched for. The comparison of the genetic clusters responsible for microcin production revealed a preserved general scheme. The clusters essentially comprise a pair of activity-immunity genes which determine antibiotic specificity and a set of microcin maturation and secretion genes which are invariably present and whose protein products are highly homologous among the different producing strains. A strict functional relationship between the maturation and secretion pathways of microcins H47, I47, and E492 was demonstrated through genetic analyses, which included heterologous complementation assays. The peptide precursors of these microcins share a maturation process which implies the addition of a catecholate siderophore of the salmochelin type. Microcins thus acquire the ability to enter gram-negative cells through the catechol receptors. In addition, they employ a common mode of secretion to reach the external milieu by means of a type I export apparatus. The results presented herein lead us to propose that chromosome-encoded microcins constitute a defined subgroup of peptide antibiotics which are strictly related by their modes of synthesis, secretion, and uptake.