REGULATION OF REPLICATION OF LAMBDA-PHAGE AND LAMBDA-PLASMID DNA AT LOW-TEMPERATURE

It was previously demonstrated that while lysogenic development of bac teriophage lambda in Escherichia coli proceeds normally at low tempera ture (20-25 degrees C), lytic development is blocked under these condi tions owing to the increased stability of the phage CII protein. This effect was proposed to be responsible for the increased stimulation of the p(E) promoter, which interferes with expression of the replicatio n genes, leading to inhibition of phage DNA synthesis. Here we demonst rate that the burst size of phage lambda cIb2, which is incapable of l ysogenic development, increases gradually over the temperature range f rom 20 to 37 degrees C, while no phage progeny are observed at 20 degr ees C. Contrary to previous reports, it is possible to demonstrate tha t p(E) promoter activation by CII may be more efficient at lower tempe rature. Using density-shift experiments, we found that phage DNA repli cation is completely blocked at 20 degrees C. Phage growth was also in hibited in cells overexpressing cII, which confirms that CII is respon sible for inhibition of phage DNA replication. Unexpectedly, we found that replication of plasmids derived from bacteriophage lambda is neit her inhibited at 20 degrees C nor in cells overexpressing cII. We prop ose a model to explanation the differences in replication observed bet ween lambda phage and lambda plasmid DNA at low temperature.