Novel mutants in the 5' upstream region of the portal protein gene 20 overcome a gp40-dependent prohead assembly block in bacteriophage T4.

The exact mechanism by which the double-stranded (ds) DNA bacteriophages incorporate the portal protein at a unique vertex of the icosahedral capsid is unknown. In phage T4, there is evidence that this vertex, constituted by 12 subunits of gp20, acts as an initiator for the assembly of the major capsid protein and the scaffolding proteins into a prolate icosahedron of precise dimensions. Assembly of the T4 initiator vertex occurs on the membrane and is facilitated by the non-structural protein gp40. gp40 apparently acts as a catalyst for the gp20 assembly and a direct interaction between gp20 and gp40 has been proposed based on the genetic evidence that second site suppressors of g40 mutants map in g20. But, surprisingly, we found that these 40bypass mutants arose not by alterations in the g20 structural gene, but by alterations in the upstream non-coding region. At least six independent bypass mutants were isolated with all except one having mutations in the non-coding region. The only exception that had a mutation in the coding region was a silent mutation, since it did not alter the amino acid sequence of gp20. The bypass mutants produced a three- to fivefold overexpression of gp20. That the gp20 overexpression is directly responsible for 40bypass was shown by a number of approaches. The overexpression was apparently due to a secondary structural change in the g20 transcript resulting in an enhanced translational initiation of g20 message. The data suggest that the regulation of portal protein gene expression is an important regulator of prohead assembly in bacteriophage T4.