Relationship between the proteins encoded by the exclusion determining locus of the IncI plasmid R144 and the cellular localization of these proteins in Escherichia coli K-12.

A region of the IncI plasmid R144, determining and controlling exclusion (exc), codes for two proteins, designated 13K and 19K after their apparent molecular weights (respectively 13,000 and 19,000). Both proteins were simultaneously affected by various mutations that resulted in exclusion deficiency. In this paper the relationship between these proteins as well as their cellular location is reported. We found no indications that the 19K protein is a precursor form of the 13K protein. Analysis of gene products of recombinant plasmids carrying exc as well as of several derivatives, however, provided a strong indication that the proteins result from overlapping genes. Besides, evidence was obtained that the 19K protein is essential for exclusion. Localization studies revealed that this protein exists in a membrane-bound form, associated at the periplasmic side of the inner membrane, and in a soluble form residing in the cytoplasm.