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Porphyromonas gingivalis is the most common organism linked to adult forms of periodontal disease. The proteome analysis of P. gingivalis cells that were placed in a mouse subcutaneous chamber revealed that ten proteins were upregulated in host tissues, whereas four proteins were downregulated. Among them, three upregulated proteins, PG1089 (DNA-binding response regulator RprY), PG1385 (TPR domain protein), and PG2102 (immunoreactive 61 kDa antigen) were chosen for further analysis. Mouse abscess model experiments revealed that the mutant strains defective in PG1089 and PG1385 were clearly less virulent, whereas the mutant defective in PG2102 was as virulent as the wild type parent strain. These results indicate that PG1089 and PG1385 proteins are involved in virulence of P. gingivalis. Using the yeast two hybrid system, PG1385 was associated with 40 proteins including four periplasmic proteins; lack of PG1385 may result in loss of physiological function of these proteins.

In this chapter, we review two topics, protein biosynthesis and thermophilic adaptation. Protein biosynthesis is of central importance for all living systems. The process of polypeptide translation and the subsequent modifications leading to the final three-dimensional protein structure are highly complex biochemical reactions that involve many components. Even in normal growth conditions, some heat-shock proteins (HSPs) bind to ribosomes and the ribosomebound nascent chain to prevent misfolding and aggregation of nascent chains. Under heat stress, the amounts of these HSPs are increased and other HSPs are newly induced and bind to the ribosomes. In addition to HSPs, constitutive components of ribosomes have chaperone activity and may contribute to ribosome assembly and biosynthesis and folding of nascent chains. It has become clear that the biosphere contains a variety of microorganisms that can live and grow in extreme environments. Thermophilic microorganisms proliferate at temperatures of more than 60°C (for hyperthermophilic microorganisms, more than 80°C). To grow at extreme conditions microorganisms require adaptive mechanisms. The results of current studies on these microorganisms provide information about the unique properties of molecular factors that impart stability to thermostable biomolecules.