Suppression of bacteriophage T7 ssb mutation with host ssb

Three new mutants of bacteriophage T7 gene 2.5, which encodes a single-stranded DNA-binding protein (ssb), were isolated and characterized. One of them, ts2.5, which showed temperature-sensitive growth, was found to have two mutations in the gene: one a missense mutation generating a Gly143-->Ser change, and the other an amber mutation at Tyr215. The other two mutants (am2.5-1 and am2.5-2) had amber mutations at Tyr15 and Ser201, respectively. None of these mutants produced a significant number of viable progeny under restrictive conditions, irrespective of whether the Escherichia coli ssb protein was functional. However, another gene 2.5 mutant, up2, which we had isolated previously, was found to be dependent on the function of host ssb for growth. Further analysis of the up2 mutation revealed that it had two additional mutations at genes 6 and 18 besides an opal mutation, op1, in gene 2.5. Neither of the suppressor mutations for the op1 mutation suppressed other gene 2.5 mutations, ts2.5 and am2.5-2. A mutant having the op1 mutation alone was unable to grow on nonsense suppressor-free hosts regardless of the presence of host ssb. These results indicate that the suppressors are specific for the op1 mutation and can make the host ssb usable during T7 phage development.