The Process of Infection with Bacteriophage φX174: XXXII. Early Steps in the Infection Process: Attachment, Eclipse and DNA Penetration

Three distinct stages are demonstrated in the process whereby φX174 invades its host: 1. (1) Attachment: the phage attach to the cell in a manner that does not irreversibly alter the phage particles. Phage attachment per se does not harm the cell. 2. (2) Eclipse: the attached phage undergo a conformational change. Some of the altered phage particles spontaneously detach from the cell (in a non-infective form) while the remainder are more tightly bound to the cell. The altered phage particles detached (spontaneously or chemically) from such complexes have at least 40% of their DNA extruded from the phage coat. It is proposed that this particle is, or derives from, a direct intermediate in the penetration of the viral DNA. 3. (3) DNA penetration: a fraction of the attached, eclipsed phage particles corresponding in number to the phage-forming units complete DNA penetration. The penetrated DNA is found in the cell as RF, and the empty phage protein coat remains firmly attached to the exterior of the cell. This step is inhibited by prior irradiation of the phage with relatively high doses of ultraviolet light. Both eclipsed phage particles and empty phage protein coats may be dissociated from infected cells; some of their properties are described