Member of a two-component system Hik34/Rre1, controlling expression of at least 20 genes in response to hyperosmotic stress (0.5 M sorbitol) or salt (0.5 M NaCl) (PubMed:15471853, PubMed:15805106, PubMed:19411329). Represses expression of heat shock genes under normal growth conditions (PubMed:15965020). Required for survival of long-term heat shock exposure (PubMed:25738257). ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Expression strongly induced by heat shock at 44 degrees Celsius for 20 minutes, after 60 minutes expression has fallen. When expressed in E.coli autophosphorylates at 18 to 30 degrees Celsius; less phosphorylation occurs at 36 and none occurs at 42 or 48 degrees Celsius. Loss or reduction of expression of about 20 genes in response to hyperosmotic stress (0.5 M sorbitol) or salt (0.5 M NaCl) stress (PubMed:15471853, PubMed:15805106, PubMed:19411329). Increased survival after heat shock from 30 to 48 degrees Celsius for 2-3 hours. Increased expression of some heat shock and photosystem-related proteins (PubMed:15965020). Cells die after 24 hours of heat shock (32 to 44 degrees Celsius). Increased expression of some heat shock proteins during normal growth (PubMed:25738257).