MODULATION OF BACTERIOPHAGE-T4 CAPSID SIZE

Bacteriophage T4 capsid assembly requires the vertex protein (gp24). M utations that bypass this requirement are found in gene 23, which prod uces the major capsid protein (gp23). The latter were used to study th e role of gp24 in head length control. We found that gp24 is no longer present in the capsids of several gp24 bypass mutants. We measured th e capsid lengths of several of these bypass mutants, because gp24 had been reported to be implicated in headlength control. One bypass mutan t (reported in 1977) produced 40-60% short headed (''petite'') phage i n the presence of wild-type amounts of gp24. The bypass mutations, whe n combined with amber mutations in gene 24, produced normal size heads in either suppressor or nonsuppressor host bacteria. When several kno wn bypass mutations were back-crossed with wild-type phage, one-third of the byp/24wt mutants isolated produced large amounts of petite phag e, indicating that the ability to produce petite phage is a general pr operly of the bypass mutations. Sequencing several of these bypass mut ants showed that those that produced petite phage contained at least o ne additional missense mutation in gene 23. This suggests that gp24 it self has no direct role in head length regulation, but that in the pre sence of bypass 24 mutations and certain easily acquired gene 23 mutat ions (called trb) the gp23-gp24 interactions can modulate head length. (C) 1996 Academic Press, Inc.