Total sequence, flanking regions, and transcripts of bacteriophage T4 nrdA gene, coding for alpha chain of...

As a part of a study of the role of bacteriophage T4-coded ribonucleoside diphosphate reductase in the switch-on and regulation of T4 DNA replication, we report the cloning and sequencing of the nrdA gene, coding for the alpha protein chain of the enzyme. The open reading frame of the nrdA gene begins 558 base pairs downstream of the 3' terminus of the td gene (thymidylate synthase). nrdB, encoding the beta chain of the enzyme, initiates 700 base pairs from the termination of nrdA. A high degree of similarity is found between the deduced amino acid sequence of the 754-residue alpha chain and the corresponding chain reported for nrdA of Escherichia coli; 56% of the residues are identical, with some segments reaching 84%. Some structural aspects of the derived alpha 2 subunit of the T4 enzyme are explored. By the S1 nuclease protection method, the RNA formed after T4 infection contains two prereplicative transcripts for nrdA, T3 and TU, and one postreplicative transcript, TL. T3 is found in low concentration. While the 5' termini of T3 and TL occur at sites near nrdA, TU apparently is a multicistronic transcript initiating farther upstream. The regulation of nrdA expression is examined in light of these findings.