Minor Coat Protein Research Articles

Electro-optic evidence for the control of the structure of bacteriophage f1 by a minor coat protein

The structures of the filamentous bacteriophage f1 and its gene 3 amber mutant R4 have been compared using electric birefringence, electric dichroism, and ultrasonic vibration. The electro-optic experiments showed that the phage particles can be oriented in an electric field. The birefringence and dichroism as a function of field strength are not the same for the A-protein mutant and for the wild-type particle. Studies using ultrasonic vibration to fragment the bacteriophage show that the stabilities of the f1 and R4 particles differ. We have interpreted the structural differences between the particles as reflecting alterations in the arrangement of the subunits of the major capsid protein, the B-protein; further, the observed suppressor dependence indicates that the subunit packing is controlled by the A-protein. Models for the mechanism of A-protein function are discussed.

The proteins of bacteriophage M13.

Particles of the small filamentous coliphage M13 contain not only the major coat protein, which is the product of phage gene 8, but also a minor coat protein, the A protein, which is the product of gene 3. The A protein has a molecular weight of approximately 70,000 daltons, is present in one copy per virion, and is responsible for phage attachment to host cells. Also associated with purified M13 particles is a minor quantity of very small proteinaceous material, but its origin as a phage-coded product has not been demonstrated. At least five phage-specific proteins, including the two coat proteins, are present in appreciable quantities in M13-infected cells. The principal phage protein synthesized is the product of gene 5, which is responsible for phage single-stranded DNA synthesis. This protein has a molecular weight of about 8,000 daltons. Its precise function in DNA synthesis is not yet known. Phage proteins are synthesized at nearly normal rates in cells in which replication of phage double-stranded DNA is blocked by gene 2 mutations. This result suggests that the initial double-stranded DNA molecule serves as the principal template, perhaps the only template, for phage messenger RNA synthesis.