The phage P221 is a hybrid between the morphologically and serologically unrelated phages P22 and P221 carries about 30% of P22 DNA containing a number of the early genes. Head and tail proteins are morphologically and serologically indistinguishable from those of The early proteins synthesized in strain Q1 after P221 infection were pulse-labelled with S and analysed by gel electrophoresis and autoradiography. These P221 early proteins were compared with early P22 proteins in an effort to detect proteins specified by the homologous region between P22 and P221. This analysis showed that nine P221 protein bands correspond to P22 protein bands. Seven of these protein bands appeared early, about 6 min after infection, and two additional protein bands appeared about 18 min after infection. The origin of these nine protein bands was also determined by protein synthesis patterns of P22 amber mutants. A large number of previously isolated P22 amber mutants were subdivided as to their location within the homologous or non-homologous region by complementation and marker rescue experiments. Four P22 amber mutants located within the homologous region were chosen for analysis of their early proteins. When the non-permissive host Q1 was infected with a P22 amber mutant in gene 24 (the gene which initiates the transcription of the P22 phage genome) no phage protein bands were observed. Similarly, when two P22 amber mutants located in gene 12 and gene 23 infected the non-permissive host Q1 no phage protein bands were detected. Mutants of the gene for endolysin showed that one specific protein band was lacking in the non-permissive host. When the amber suppressor host Q was infected with any of the above amber mutants all protein bands were found. By the use of an amber mutant in gene 3 of the P22 late gene region located outside the P22–P221 homologous region, we showed that mutations in the non-homologous region did not affect the synthesis of these nine proteins. The genetic origin of proteins found in mature P221 particles was determined by subjecting proteins of purified P221, P22 and phage particles to analysis by SDS-gel electrophoresis. It was concluded that P221 and share the same head, tail and internal proteins in mature phage particles, whereas none of the protein components of these mature phage particles is the same as those of P22 particles.

Keyword(s): bacteriophage , genetics , P22 and Salmonella

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