A common sequence motif among prokaryotic DNA primases

DNA polymerases cannot start a chain de novo and must rely on priming devices. DNA primases provide free 3'-hydroxyl termini by the synthesis of short oligonucleotides that are base-paired with the template DNA. Bacteria, several bacteriophages and plasmids of certain incompatibility groups encode genes specifying DNA primases. Recently, two different sets of amino acid sequence comparisons of several well-characterized DNA primases were published. Combining these data demonstrates the existence of a common motif in the published prokaryotic DNA primase sequences. It is interesting that two neighboring amino acid residues, namely E and G are present in all these sequences, whereas the adjacent site chains only share some chemical properties as indicated by the consensus sequence. Exchange of the glutamic acid residue by site-directed mutagenesis (E to Q) results in complete loss of priming activity for the RP4 TraC sub(2) protein and the phage P4 alpha protein. Since dramatic structural changes are not expected in the mutant proteins, the glutamic acid residue apparently plays an important role in the catalytic activity of at least two of the enzymes.