POLYMORPHISM OF BACTERIOPHAGE-T7

For viruses made of nucleic acid and protein, the structure of the pro tein outer shell. has, in the past, been found to be uniquely determin ed by the viral genome. However, here, non-denaturing agarose gel elec trophoresis of bacteriophage T7 reveals two states of the mature T7 ca psid; the conditions of growth are found to alter the population by T7 of these two electrophoretically-defined states. Both stales have bee n previously observed for a genetically altered T7 and they are observ ed here for wild-type T7. The average electrical surface charge densit y of a bacteriophage particle (delta) determines its state; the delta of particles in both states is negative. For a given condition of grow th, the population of these two states is influenced by the extent to which the major T7 outer shall protein, p10A, is accompanied by its mi nor readthrough variant, p10B. Comparison of the two electrophoretic s tates reveals the following. (1) No difference in radius is present in the outer shell (+/-2%). (2) As the pH of electrophoresis is either i ncreased or decreased from neutrality, the state becomes more highly p opulated for which delta is greater in magnitude (state 1). By changin g the pH, some T7 particles are made to change state. (3) Particles in state 1 adsorb less quickly to host cells than do the particles in th e alternative state (state 2). This latter observation suggests the hy pothesis that state 1 evolved to reduce the probability of re-initiati ng an infection when conditions are not favorable for growth. This hyp othesis is supported by the observation that, as conditions of growth become apparently more unfavorable, progeny increasingly populate stat e 1. (C) 1997 Academic Press Limited.