Degradation of mutant initiator protein DnaA204 by proteases ClpP, ClpQ and Lon is prevented when DNA is SeqA-free.

Abstract

A mutant form of the Escherichia coli replication initiator protein, DnaA204, is unstable. At low growth rates, the dnaA204 mutant cells experience a limitation of initiator protein and grow with reduced initiation frequency and DNA concentration. The mutant DnaA protein is stabilized by the lack of SeqA protein. This stabilization was also observed in a dam mutant where the chromosome remains unmethylated. Since unmethylated DNA is not bound by SeqA, this indicates that DnaA204 is not stabilized by the lack of SeqA protein by itself, but rather by lack of SeqA complexed with DNA. Thus the destabilization of DnaA204 may be due either to interaction with SeqA-DNA complexes or changes in nucleoid organization and superhelicity caused by SeqA. The DnaA204 protein was processed through several chaperone/protease pathways. The protein was stabilized by the presence of the chaperones ClpA and ClpX and degraded by their cognate protease ClpP. The dnaA204 mutant was not viable in the absence of ClpY, indicating that this chaperone is essential for DnaA204 stability or function. Its cognate protease ClpQ, as well as Lon protease, degraded DnaA204 to the same degree as ClpP. The chaperones GroES, GroEL and DnaK contributed to stabilization of DnaA204 protein.