Effect of multiplicity of infection on transcription in Escherichia coli cells infected by bacteriophage lambda.

The effect of multiplicity of infection (m.o.i.) on transcription was studied by infecting Escherichia coli with bacteriophage lambda cI47, lambda cI47O29P3 and lambda cI857cro27P3. DNA-RNA hybridization with lambda cI47 l-strand DNA, phi 80imm lambda r-strand DNA, and lambda imm80 r-strand DNA were used to measure mRNA transcription from the l-strand, the r-strand of the early x-P-Q region and the late A-J-b2 region of lambda bacteriophage respectively. In lambda cI47cro+O-P--infected cells, transcription from the l-strand, r-strand of the early x-P-Q region and the late A-J-b2 region all decreased with increasing m.o.i. The response in the x-P-Q region was less marked than in other regions, but the pattern looked similar to that described above. When phage DNA replication was permitted, as in the case of lambda cI47, the response was similar to that observed in lambda cI47cro+O-P--infected cells, but the level of transcription was increased two- or threefold. In lambda cI857cro-P--infected cells, the leftward transcription and the rightward transcription from the early x-P-Q region and the late A-J-b2 region all increased with increasing m.o.i., but the extent of change was less drastic than with lambda cro+. This result demonstrated clearly that the decrease in transcription from various regions at increasing m.o.i. of lambda cro+ was due to the inhibitory action of the cro gene product. The results obtained with cro- strongly support the view that gene dosage is a significant controlling factor for the extent of gene expression.