Analysis of the mechanism of cell lysis by RNA phage SP

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A2 protein is one of the proteins which constitute the phage particle. In the case of phage *beta which is closely related to phage SP, the overproduction of the A2 protein causes the cell lysis when the A2 gene is cloned and expressed on the plasmid. In order to investigate the participation of the A2 gene of phage SP in the lysis, we constructed the clones harboring the A2 gene downstream from the lac or the cro promoter of the plasmid pUC8 or pSP857CRO, respectively. But the cells did not lyse upon induction of the A2 gene in every case, although the cell growth was suppressed in the latter case. On the other hand, when the antisense RNAs against the A2 gene were expressed during SP phage infection, the cell lysis was prevented, however, the phage assembly proceeded normaly within the cells. These results suggest that the A2 gene is responsible for the cell lysis and that phage assembly and lysis are distinct functions of the A2 protein.When we carried out PCR using RNAs prepared from the uninfected cells as templates and the oligonucleotides identical to the A2 gene as primers, we found that the distinct length of DNAs were amplified. The nucleotide sequences of these DNAs were found to be homologous to that of the A2 cDNA.We propose that there are the segments similar to the A2 gene on the host chromosome.