Clear plaque mutants (Cf1c) isolated from the temperate filamentous phage Cf1t occurred at a frequency of approximately 10-3. The phage yield from Cf1c-infected cells was higher than that from Cf1t-infected cells. Results of spot complementation tests implied that the turbid plaque phenotype is dominant. DNA fragment substitution studies indicated that a Ncol/Kpnl fragment of 591 by was responsible for the determination of plaque turbidity. Sequence data from four Cf1c isolates revealed base pair alterations and a deletion located in the upstream region of an open reading frame (ORFII) which might encode a 18.2-kDa protein. When the ORFII in Cfi t was disrupted by a frameshift mutation, this recombinant phage formed clear plaques. These observations suggest that ORFII may participate in the formation of turbid plaques. ORFII does not show significant homology with the sequence of f1 gpII, gpV, or other known phage proteins.
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