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 wild-type 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, p _R and p _L, were found to be slightly dependent on the host growth rate. However, when p _R 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.