hipA, Serine/threonine-protein kinase toxin HipA
Enzyme Classification 18.104.22.168
Also Known as
Toxic component of a type II toxin-antitoxin (TA) system, first identified by mutations that increase production of persister cells, a fraction of cells that are phenotypic variants not killed by antibiotics, which lead to multidrug tolerance (PubMed:6348026, PubMed:8021189, PubMed:16707675, PubMed:26222023). Persistence may be ultimately due to global remodeling of the persister cell's ribosomes (PubMed:25425348). Phosphorylates Glu-tRNA-ligase (AC P04805, gltX, on 'Ser-239') in vivo (PubMed:24095282, PubMed:24343429). Phosphorylation of GltX prevents it from being charged, leading to an increase in uncharged tRNA(Glu). This induces amino acid starvation and the stringent response via RelA/SpoT and increased (p)ppGpp levels, which inhibits replication, transcription, translation and cell wall synthesis, reducing growth and leading to persistence and multidrug resistance (PubMed:24095282, PubMed:24343429, PubMed:25848049). Once the level of HipA exceeds a threshold cells become dormant, and the length of dormancy is determined by how much HipA levels exceed the threshold (PubMed:20616060). The hipA7 mutation (a double G22S D291A mutation) leads to increased generation of persister cells (cells that survive antibiotic treatment) probably by entering into a dormant state, as well as cold-sensitivity (PubMed:14622409, PubMed:16707675). Wild-type cells produce persisters at a frequency of 10(-6) to 10(-5) whereas hipA7 cells produce about 100-fold more persisters (PubMed:14622409, PubMed:16707675, PubMed:25425348). hipA7 decreases the affinity for antitoxin HipB, leading to increased HipA levels and persistence (PubMed:20616060); depending on the protein level, can be toxic enough to reduce cell growth or even kill cells. Generation of persister cells requires (p)ppGpp as cells lacking relA or relA/spoT generate fewer or no persister cells respectively compared to hipA7 (PubMed:14622409, PubMed:25848049). Generation of persister cells by HipA also requires mRNA interferase toxins (such as MazF, RelE or YafO) and Lon protease; a strain deleted for 10 type II TAs does not produce persisters when HipA (or HipA7) is expressed, nor does a lon deletion strain or bacteria with alterations of polyphosphate levels, although levels of (p)ppGpp increase (PubMed:25848049). The toxic effect of HipA is neutralized by its cognate antitoxin HipB (PubMed:20616060). Also neutralized by overexpression of gltX (PubMed:24343429, PubMed:28430938). With HipB acts as a corepressor for transcription of the hipBA promoter (PubMed:8021189); binding of HipA-HipB to DNA induces a 70 degree bend (PubMed:19150849, PubMed:26222023). This brings together and dimerizes 2 HipA molecules, which distorts the promoter region, preventing sigma-factor binding; additionally HipA and HipB would physically prevent RNA core polymerase from contacting the -35 promoter box (PubMed:26222023). May play a role in biofilm formation (PubMed:23329678). Forms a HipA(2)HipB(2) heterotetramer which can interact with a single operator on DNA (PubMed:19150849). When 2 operators are present each HipB dimer contacts 1 HipA molecule, which are brought together by the DNA bend and dimerize, blocking the HipA active site and inactivating its toxic activity (PubMed:26222023). Mutations present in allele hipA7 (G22S and D291A) decrease the affinity of HipA for HipB (PubMed:20616060).