EntG activity of Escherichia coli enterobactin synthetase

The last steps in the biosynthesis of the Escherichia coli siderophore enterobactin (Ent) are carried out by Ent synthetase, a multienzyme complex believed to be composed of the entD, -E, -F, and -G products (EnTD to -G). However, sequencing data showed that there is no separate entG gene and, unlike EntD to -F, no distinct EntG polypeptide has been identified. In this study, genetic, biochemical, and immunological approaches were used to study the anomalies associated with EntG activity. Two plasmids, pJS43 and pJS100, were isolated that had mutations resulting in truncated EntB proteins; both had the phenotype EntB⁺ EntG^-. PJS43 had a Tn5 inserted 198 bp from the entB termination codon, and pJS100 had the last 25 codons of entB deleted. Plasmids isolated with Tn5 insertions in the 5' half of entB had the phenotype EntB^- EntG⁺. These latter Tn5 mutation were EntB^- EntG^- when moved to the bacterial chromosome. Polyclonal antiserum was prepared and shown to react only with intact EntB in Western immunoblots. Addition of anti-EntB antiserum to Ent synthetase assays resulted in complete inhibition of enzyme activity, whereas preimmune serum had no effect. Lastly, AN462, the type strain for entG which was derived by Mu insertion and which has the phenotype Entb^-G^-A^- was characterized. Southern blot data showed a Mu insertion, presumably with polar effects, in the vicinity of the 5' end of entB. In summary, EntG activity was found to be encoded by entB 3' terminus. The evidence, while not rigorously eliminating the possibility that a separate EntG polypeptide exists, strongly supports the idea that EntB is a bifunctional protein.