Impaired expression of certain prereplicative bacteriophage T4 genes explains impaired T4 DNA synthesis in Escherichia coli rho (nusD) mutants.
The Escherichia coli rho 026 mutation that alters the transcription termination protein Rho prevents growth of wild-type bacteriophage T4. Among the consequences of this mutation are delayed and reduced T4 DNA replication. We show that these defects can be explained by defective synthesis of certain T4 replication-recombination proteins. Expression of T4 gene 41 (DNA helicase/primase) is drastically reduced, and expression of T4 genes 43 (DNA polymerase), 30 (DNA ligase), 46 (recombination nuclease), and probably 44 (DNA polymerase-associated ATPase) is reduced to a lesser extent. The compensating T4 mutation goF1 partially restores the synthesis of these proteins and, concomitantly, the synthesis of T4 DNA in the E. coli rho mutant. From analyzing DNA synthesis in wild-type and various multiply mutant T4 strains, we infer that defective or reduced synthesis of these proteins in rho 026-infected cells has several major effects on DNA replication. It impairs lagging-strand synthesis during the primary mode of DNA replication; it delays and depresses recombination-dependent (secondary mode) initiation; and it inhibits the use of tertiary origins. All three T4 genes whose expression is reduced in rho 026 cells and whose upstream sequences are known have a palindrome containing a CUUCGG sequence between the promoter(s) and ribosome-binding site. We speculate that these palindromes might be important for factor-dependent transcription termination-antitermination during normal T4 development. Our results are consistent with previous proposals that the altered Rho factor of rho 026 may cause excessive termination because the transcription complex does not interact normally with a T4 antiterminator encoded by the wild-type goF gene and that the T4 goF1 mutation restores this interaction.