Minicells containing the ColE1 plasmid and derivatives of it that contain a Tn3 insertion and deletions of the plasmid were shown to synthesize a variety of polypeptides: (i) a 56,000-dalton polypeptide was found that is colicin E1. (ii) A 15,000-dalton polypeptide is involved in plasmid DNA relaxation. (iii) A 14,000-dalton polypeptide may be the colicin immunity protein. (iv) A correlation between the location of various Tn3 insertions and the proteins synthesized from the plasmid DNA was found that suggests the locations of promoter sites in the plasmid and directions of transcription in two regions of the plasmid. (v) Polypeptides associated with the presence of Tn3 were identifiable. (vi) The nature and extent of gene expression of deletion mutants only generally correlates with the extent of the deletions.