Two classes of missense mutations in the bacteriophage T4 gene coding for the major head protein produce phage with different length heads. The pt (petite) mutations produce phage with normal, intermediate, and isometric heads, whereas ptg (petite and giant) mutations also produce greatly elongated (giant) heads. DNA from petite, normal, and giant particles was clearly resolved by discontinuous rotating gel electrophoresis, and several new species of headful length DNA were found. These results confirm the idea that the major stop points for head length regulation are at Q = 13, 17, and 21, and also show that minor stop points exist at Q = 16, 18 and 20. The existence of these well-defined classes of DNA that correlate with capsid structure suggest that a structural relationship between the scaffold protein and the capsid protein determines head length and thus DNA length.
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