Study on Two-Component Signal Transduction System in an Intracellular Symbiotic Bacterium

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Symbionin, a GroEL homologous molecular chaperone produced by Buchnera, an intracellular symbiotic bacterium of aphids, is not only a molecular chaperone but also a novel phosphocarrier protein, suggesting that this protein plays a key role in 'the signal transducing system specific to the bacterium living in an intracellular environment. This promoted me to look into Buchnera phosphocarrier proteins such as "sensor molecule or histidine protein kinase (HPK)" of the two-component signal transducing system and "histidine-containing phosphocarrier protein (HPR) " of phosphotransferase system (PTS) that may be shared in common with other free-living bacteria, and the following results were obtained.1. A 25-kDa protein that is able to accept the phosphoryl moiety of phosphorylated symbionin was identified as histone H1. Buchnera histone H1 is extracted with 5% PCA like eukaryotic histones and also retains the DNA-binding activity. The gene for Buchnera histone H1 was cloned and the full amino acid sequence was deduced from its nucleotide sequence. This is the first report of the prokaryotic histone H1. In contrast, no evidence was obtained for the presence of sensor kinases of the two-component system in Buchnera, which are generally found in many free-living bacteria. It is possible that the lack of sensor kinases in Buchnera genome is compensated for by multifunctional symbionin in this symbiotic bacterium.2. I successfully identified ptsH-ptsl-ccr and provide the evidence that these three genes are actively expressed in Buchnera cells. Also, it was demonstrated that "Enzyme I" is auto-catalytically phopshorylated by phosphoenolpyruvate (PEP), but not by ATP, suggesting that phosphotransferase system (PTS) is also actively functioning in Buchnera cells.