Plastids are evolutionary descendants of cyanobacteria and retain many proteins of cyanobacterial origin including histidine kinases (HKs). Histidine kinases form a major group of protein kinases in cyanobacteria but a minor group in angiosperms; no HK has been detected in plant plastids so far. This raises the question: have higher plant plastids retained some cyanobacterial His/Asp regulatory systems or the latter have been replaced with Ser/Thr or Tyr protein kinases more typical for eukaryotes? To answer this question H1 antiserum against conservative phospho-acceptor motif of HKs was raised and applied for the analysis of different chloroplast fractions. In maize, three polypeptides with apparent molecular masses about 26, 27.5, and 28 kDa were revealed in thylakoid membranes. The results of in organello phosphorylation suggest that these proteins may be His-phosphorylated. Polypeptides with similar molecular masses were revealed in various mono- and dicotyledonous plants. Bioinformatic analysis demonstrated that in angiosperms these polypeptides might result from alternative transcription initiation and/or alternative processing of mRNAs of genes encoding well-known HKs. Besides, the genome of the moss Physcomitrella patens contains much more genes that could code for plastid HKs. The data obtained let us suppose that plastids of contemporary plants have HK(s) that could be involved in regulation of plastid gene expression.
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