Infection of Synechococcus cedrorum by the cyanophage AS-1M : I. Ultrastructure of infection and phage assembly

Abstract

The replication of cyanophage ȦS-1M after infection of the blue-green alga Synechococcus cedrorum was monitored by electron microscopy. One-step growth and intracellular growth experiments were performed at high multiplicity of infection (m.o.i.) and the basic characteristics of phage growth determined. The eclipse period lasts until 3–4 hr after infection, the latent period continues until 8 hr, and complete lysis of the culture terminates about 12 hr after infection. The average burst size at high m.o.i. was about 40 plaque-forming units (PFU) per cell. Samples for electron microscopy were taken at 2, 4, 6, 8 and 10 hr after infection. The first noticeable evidence of phage infection was a degradation of the host nucleoplasm, which was manifest as a distinct lightening of the nucleoplasm and aggregation of ribosomes near the photosynthetic lamellae. All further phage synthesis takes place within the nucleoplasm. At 4 hr after infection, a large amount of fibrillar material, presumably the vegetative phage DNA, is evident. Beginning at this time, but particularly at 6 hr after infection, many phage precursors in the process of assembly can be seen; these include empty-head vesicles and “helical” structures which appear to be heads in the process of packaging the DNA. Based on the structures seen in the EM, we speculate that empty heads form first and are then filled with DNA. The completed capsid is then joined to the tail.