 If two lox sites are present (one in each terminally redundant end) the cyclization is carried out by the cre recombinase.Once the complete virions are assembled, the host cell is lysed, releasing the viral particles.P1 was discovered in 1951 by Giuseppe Bertani in Salvador Luria's laboratory, but the phage was little studied until Ed Lennox, also in Luria's group, showed in 1954–5 that it could transduce genetic material between host bacteria.