ClpP/ClpX-mediated degradation of the bacteriophage λ O protein and regulation of λ phage and λ plasmid replication

Abstract

The O protein is a replication initiator that binds to the oriλ region and promotes assembly of the bacteriophage λ replication complex. This protein, although protected from proteases by other elements of the replication complex, in a free form is rapidly degraded in the host, Escherichia coli, by the ClpP/ClpX protease. Nevertheless, the physiological role of this rapid degradation remains unclear. Here we demonstrate that the copy number of plasmids derived from bacteriophage λ is significantly higher in wild-type cells growing in rich media than in slowly growing bacteria. However, λ plasmid copy number in bacteria devoid of the ClpP/ClpX protease was not dependent on the bacterial growth rate and in all minimal media tested was comparable to that observed in wildtype cells growing in a rich medium. Contrary to λ plasmid replication, the efficiency of lytic growth of bacteriophage λ was found to be dependent on the host growth rate in both wild-type bacteria and clpP and clpX mutants. The activities of two major λ promoters operating during the lytic development, pR and pL, were found to be slightly dependent on the host growth rate. However, when pR activity was significantly decreased in the dnaA mutant, production of phage progeny was completely abolished at low growth rates. These results indicate that the O protein (whose level in E. coli cells depends on the activity of ClpP/ClpX protease) is a major limiting factor in the regulation of λ plasmid replication at low bacterial growth rates. However, this protein seems to be only one of the limiting factors in the bacteriophage λ lytic development under poor growth conditions of host cells. Therefore, it seems that the role of the rapid ClpP/ClpX-mediated proteolysis of the O protein is to decrease the efficiency of early DNA replication of the phage in slowly growing host cells.