Two new early bacteriophage T4 genes, repEA and repEB, that are important for DNA replication initiated from origin E

Two new, small, early bacteriophage T4 genes, repEA and repEB, located with in the origin E (oriE) region of T4 DNA replication, affect functioning of this origin. An important and unusual property of the oriE region is that i t is transcribed at early and late periods after infection, but in opposite directions (from complementary DNA strands). The early transcripts are mRN As for RepEA and RepEB proteins, and they can serve as primers for leading- strand DNA synthesis. The late transcripts, which are genuine antisense RNA s for the early transcripts, direct synthesis of virion components. Because the T4 genome contains several origins, and because recombination can bypa ss a primase requirement for retrograde synthesis, neither defects in a sin gle origin nor primase deficiencies are lethal in T4 (Mosig et al., FEMS Mi crobiol. Rev. 17:83-98, 1995). Therefore, repEA and repEB 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 two repE gen es, we constructed nonsense mutations in each of them and combined them wit h the motA mutation sip1 that greatly reduces initiation from other origins . As expected, T4 DNA synthesis and progeny production were severely reduce d in the double mutants as compared with the single motA mutant, but early transcription of oriE was reduced neither in the motA nor in the repE mutan ts. Moreover, residual DNA replication and growth of the double mutants wer e different at different temperatures, suggesting different functions for r epEA and repEB. We surmise that the different structures and protein requir ements for functioning of the different origins enhance the flexibility of T4 to adapt to varied growth conditions, and we expect that different origi ns in other organisms with multiorigin chromosomes might differ in structur e and function for similar reasons.