Plaque forming specialized transducing phage P1: isolation of P1CmSmSu, a precursor of P1Cm.

Abstract

E. coli strains lysogenic for various types of P1-R hybrids were isolated. These carry all the essential genes for vegetative phage production and lysogenization including P1 immunity and P1 incompatibility, together with drug resistance genes derived from the R plasmid NR1. In particular, P1Cm and P1CmSmSu derivatives were studied. When strains lysogenic for these phages were induced in the absence of helper phage, yields of phage particles as high as with wild type P1 were obtained. All P1Cm phages isolated were of plaque forming type and usually every plaque contained Cmr lysogens. Lysates of P1CmSmSu lysogens transduced CmrSmrSur at high frequency and they formed plaques with an efficiency of 10-4 to 10-2 per phage particle. Only a minority of these plaques contained drug resistant bacteria. CmrSmrSur transductants isolated from bacteria infected at a high multiplicity with phage P1CmSmSu were lysogens for the original P1CmSmSu. In contrast, CmrSmrSur transductants isolated after infection at low multiplicity appeared to carry the CmrSmrSur markers integrated into the host chromosome. The results described suggest that P1CmSmSu prophages carry the resistance genes transposed into the P1 genome without in principle causing a loss of essential gene functions. However, since these prophages are longer than the wild type P1 genome, the DNA packaged into phage particles has a reduced redundancy which seriously affects the reproduction and lysogenization abilities.