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

The O protein is a replication initiator that binds to the ori lambda regio n and promotes assembly of the bacteriophage lambda replication complex. Th is protein, although protected from proteases by other elements of the repl ication complex, in a free form is rapidly degraded in the host, Escherichi a coil, by the ClpP/ClpX protease. Nevertheless, the physiological role of this rapid degradation remains unclear. Here we demonstrate that the copy n umber of plasmids derived from bacteriophage lambda is significantly higher in wild-type cells growing in rich media than in slowly growing bacteria. However, lambda plasmid copy number in bacteria devoid of the ClpP/ClpX pro tease was not dependent on the bacterial growth rate and in all minimal med ia rested was comparable to that observed in wildtype cells growing in a ri ch medium. Contrary to lambda plasmid replication, the efficiency of lytic growth of bacteriophage lambda 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 lambda promoters operating during the lytic development, p(R) and p(L), were found to be slightly dependent on the host growth rate. Howe ver, when p(R) activity was significantly decreased in the dnaA mutant, pro duction of phage progeny was completely abolished at low growth rates. Thes e 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 r egulation of lambda plasmid replication at low bacterial growth rates. Howe ver, this protein seems to be only one of the limiting factors in the bacte riophage lambda lytic development under poor growth conditions of host cell s. Therefore, it seems that the role of the rapid ClpP/ClpX-mediated proteo lysis of the O protein is to decrease the efficiency of early DNA replicati on of the phage in slowly growing host cells.