Identification, purification and partial sequence of four Thermus thermophilus 5S rRNA binding proteins

In view of the increasing interest in heat stable proteins of thermophilic bacteria that interact with nucleic acids, we have identified four 5S rRNA binding proteins from the extreme thermophile Thermus thermophilus. The proteins were purified under stringent conditions on a 5S rRNA affinity column and were assigned by 2D-gelelectrophoresis. The same proteins were found in several 5S rRNA ribonucleoprotein complexes that can be separated from 50S ribosomal subunits under mild denaturing conditions. N-terminal sequencing of the purified proteins revealed that only the second largest protein (TthL5, 21kd) has a significant similarity of 95% to an E. coli 5S rRNA binding protein, EcL5. A marginal similarity of about 30% was found between the next smaller protein (TL24, 15kd) and its potential counterpart, EcL18. The largest protein (TL7, 27kd) seems to be distantly related to TthL5. Surprisingly, the smallest identified protein (TL29, 13kd) is identical to the Th. thermophilus DNA binding protein II, indicating a potentially dual function of this protein.