The products of gene I and the overlapping in-frame gene XI are required for filamentous phage assembly.

The class I filamentous bacteriophage are non-lytic single-stranded DNA phage, which are assembled at the cell envelope as they are extruded from the Gram-negative bacteria, Escherichia coli. The process requires the products of the phage genes I and IV, which reside in the inner and outer membrane, respectively, and are not present in the mature phage particle. Gene I encodes two proteins, the full length 348-residue pI and a smaller pI*, which this report shows is the result of an internal translation initiation event at methionine codon 241. Both pI and pI* are shown to be required for phage assembly. Therefore, pI* can be considered the product of an additional phage gene, XI, which is a separate in-frame gene that overlaps gene I. Both proteins contain a 13-residue region adjacent to the cytoplasmic face of the inner membrane that probably exists as a positively charged amphiphilic helix. Although this region is not required for membrane insertion of pI and pI*, it is shown to be required for phage assembly. Oligonucleotide-directed mutagenesis of this region, which removes positive charges or alters the hydrophobic face of the putative helix, renders pI and pI* unable to function in phage assembly. This region of pI and pI* is highly homologous in structure to the carboxyl-terminal 11 amino acids of pVIII, the main coat protein, which also reside adjacent to the cytoplasmic face of the inner membrane.