Thermus thermophilus HB8 is an extreme thermophilic eubacrium, which grows at 50-80 degrees C. Transfer RNA molecules in T. thermophilus HB8 contains modified nucleosides such as Gm18, m(7)G46, m(5)s(2)U54, Psi55, and m(1)A58. Recently, all responsible genes for these modifications have been identified. To clarify the relationship of the amounts of the RNA modification enzymes (proteins) and culture conditions, we investigated the activities and quantities of the proteins. In this meeting, we report the amount changes of tRNA (Gm18) methyltransferase [TrmH] in T. thermophilus HB8 cells cultured at 52, 67, 75, and 79 degrees C. 60 mug of total proteins in the crude extract from cells cultured at 67 degrees C contained about 10-30 ng of the TrmH protein. Thus, the content of the TrmH protein was estimated to be around 1/5000 of the total proteins. Through the log phase, the content of the TrmH protein was not changed obviously. However, in the stationary phase, the content of the TrmH protein was slowly reduced. The TrmH contents in the cells cultured at 75 or 79 degrees C were similar to that at 67 degrees C. However, the content of the TrmH protein in the extract from 52 degrees C cultured cells slightly decreased as compared with that in the 67 degrees C cultured cells.