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 DN A phage, which are assembled at the cell envelope as they are extruded from the Gram-negative bacteria, Escherichia coli. The process requir es 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 p hage particle. Gene I encodes two proteins, the full length 348-residu e pI and a smaller pI, which this report shows is the result of an in ternal translation initiation event at methionine codon 241. Both pI a nd 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 1 3-residue region adjacent to the cytoplasmic face of the inner membran e that probably exists as a positively charged amphiphile helix. Altho ugh this region is not required for membrane insertion of pI and pI, it is shown to be required for phage assembly. Oligonucleotide-directe d 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 hom ologous 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.