Treatment of T3 with various denaturing agents produced distinct components which were identified in the ultracentrifuge. Combining this analysis with electron microscopy made it possible to identify the capsid and nucleocapsid, and a fragment having an = 105 ± 10S. This fragment appeared spherical in the electron microscope and had dimensions between 12 nm and 16 nm. It was tentatively identified as the tail. The mol. wt. of the 105S particle and the empty head (200S) were calculated to be 2.3 × 10 and 21.7 × 10, respectively. Adding these weights to the estimated weight for DNA (25 × 10, Lang & Coates, 1968) gave a total weight for the phage of 49 × 10, equal to the mol. wt. obtained by Swaby (1959). In addition to these large fragments, two peptides were examined, one had a mol. wt. of 9300 ± 1400 in 6 -GuHCl (1.5S) and appeared to be a single chain; the other, released when a suspension of phage was diluted, had a mol. wt. of less than 10000. The origins of the two peptides remain largely speculative, but in view of the marked associative properties of the 1.5S peptide and its detection only when the head was destroyed, it is likely that it was a binding fraction in the head of the phage. The second peptide may be more intimately associated with the DNA of T3.


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