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Two new, small, early bacteriophage T4 genes,repEA andrepEB , located within the origin E (oriE ) region of T4 DNA replication, affect functioning of this origin. An important and unusual property of theoriE region is that it is transcribed at early and late periods after infection, but in opposite directions (from complementary DNA strands). The early transcripts are mRNAs for RepEA and RepEB proteins, and they can serve as primers for leading-strand DNA synthesis. The late transcripts, which are genuine antisense RNAs for the early transcripts, direct synthesis of virion components. Because the T4 genome contains several origins, and because recombination can bypass a primase requirement for retrograde synthesis, neither defects in a single origin nor primase deficiencies are lethal in T4 (Mosig et al., FEMS Microbiol. Rev. 17:83–98, 1995). Therefore,repEA andrepEB were expected and found to be important for T4 DNA replication only when activities of other origins were reduced. To investigate the in vivo roles of the tworepE genes, we constructed nonsense mutations in each of them and combined them with themotA mutationsip1 that greatly reduces initiation from other origins. As expected, T4 DNA synthesis and progeny production were severely reduced in the double mutants as compared with the singlemotA mutant, but early transcription oforiE was reduced neither in themotA nor in therepE mutants. Moreover, residual DNA replication and growth of the double mutants were different at different temperatures, suggesting different functions forrepEA andrepEB . We surmise that the different structures and protein requirements for functioning of the different origins enhance the flexibility of T4 to adapt to varied growth conditions, and we expect that different origins in other organisms with multiorigin chromosomes might differ in structure and function for similar reasons.

Two New Early Bacteriophage T4 Genes, repEA and repEB , That Are Important for DNA Replication Initiated from Origin E