The amber lesion of am8H1, the only conditional lethal mutant in a filamentous phage coat protein gene, lies two codons after the signal peptidase cleavage site (Boeke & Model, 1979). We sequenced the DNA of 15 independently isolated pseudorevertants of this mutant. We studied the production of unprocessed and processed coat protein in pseudorevertant-infected cells and in amber mutant-infected suppressor strains. These studies show that serine, glutamine, tyrosine or leucine residues can replace the glutamic acid residue found in the wild-type coat protein at position 2. Reversion to tyrosine or leucine was always accompanied by a second mutation, which leads to the replacement of asparagine by aspartic acid at position 12. Leucine and, to a lesser extent, tyrosine seem to inhibit processing since pre-coat protein accumulates in the infected cells. Filamentous phage particles were shown to migrate on agarose gels with a mobility that reflects the charge of the "external", N-terminal domain of their major coat protein.
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