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The multiplication of bacteriophage T7 is blocked in Escherichia coli M. The genetic determinant of this ability (groM) to inhibit T7 growth was transferred to an E. coli K-12 recipient by means of conjugation. We determined at which precise step T7 maturation is blocked. Phage-directed protein and DNA synthesis as well as degradation of host DNA were not qualitatively affected. Instead of infective phages, only preheads were produced. These, however, were maturable in vitro. The newly synthesized phage DNA accumulated in a concatemeric form and matured from its tetrameric or longer forms (very fast sedimenting DNA) only into its dimeric form (fast-sedimenting DNA) or longer forms. The following step, i.e., the maturation of the dimeric to unit-length DNA, was not observed. Since the concatemeric form of T7 DNA accumulated in spite of the presence of maturable preheads, it is likely that the maturation process was blocked at the level of DNA packaging. As intermediates in the packaging process, we found some prehead-DNA complexes. We interpreted these as true assembly intermediates (or breakdown products thereof), since the attached DNA was still in its concatemeric form. This shows that the very first DNA packaging step, the binding of the progeny DNA to the preheads, was obviously not blocked. Rather, a later step, such as the filling of the preheads with T7 DNA or the stabilization of completely packaged particles (i.e., the final cutting of the concatemers into unit-size length), was inhibited.

Involvement of the bacterial groM gene product in bacteriophage T7 reproduction. I. Arrest at the level of DNA packaging