Toxic component of a type II toxin-antitoxin (TA) system (PubMed:18398006, PubMed:24141193). Phosphorylates EF-Tu on 'Thr-383' in vitro; can use ATP or GTP (PubMed:24141193). Also dephosphorylates in the presence of ADP or GDP. Can only phosphorylate before the ternary aminoacyl-tRNA EF-Tu-GTP complex is formed; interaction with cognate antitoxin phd prevents phosphorylation and dephosphorylation of EF-Tu. Overexpression results in inhibition of growth in liquid cultures and a decrease in colony formation by inhibiting translation, stabilizing mRNA and polysomes; these effects are overcome by concomitant expression of antitoxin phd. Binds 70S ribosomes and the 30S ribosomal subunits, the binding site is the same as for the antibiotic hygromycin B (PubMed:18398006). Bacteriophage P1 lysogenizes bacteria as a low-copy number plasmid; doc and phd proteins function in unison to stabilize plasmid number by inducing a lethal response to P1 plasmid prophage loss (PubMed:8411153). Overexpression of doc can induce the mRNA interferase activity of host RelE in vivo (PubMed:18757857). Antitoxin phd binds to its own promoter repressing its expression; toxin doc acts as a corepressor or derepressor depending on the ratio, repressing or inducing expression. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with cognate antitoxin phd, the exact ratio of doc:phd varies from 1:1 to 1:3. Interaction with phd prevents kinase or phosphorylase activity on EF-Tu. Interacts with the 30S ribosomal subunit. The concentration of antitoxin phd in P1 lysogens is far greater than that of the toxin it antagonizes. Such an excess may assure the well-being of carriers of the addicting plasmid.