The structure of the archaebacterial ribosomal protein S7 and its possibleinteraction with 16S rRNA

Ribosomal protein S7 is one of the ubiquitous components of the small subun it of the ribosome. It is a 16S rRNA-binding protein positioned close to th e exit of the tRNA, and it plays a role in initiating assembly of the head of the 30S subunit. Previous structural analyses of eubacterial S7 have sho wn that it has a stable alpha -helix core and a flexible beta -arm. Unlike these eubacterial proteins, archaebacterial or eukaryotic S7 has an N-termi nal extension of approximately 60 residues. The crystal structure of S7 fro m archaebacterium Pyrococcus horikoshii (PhoS7) has been determined at 2.1 Angstrom resolution. The final model of PhoS7 consists of six major alpha - helices, a short 3(10)-helix and two beta -stands. The major part (residues 18-45) of the N-terminal extension of PhoS7 reinforces the a-helical core by well-extended hydrophobic interactions, while the other part (residues 4 6-63) is not visible in the crystal and is possibly fixed only by interacti ng with 16S rRNA. These differences in the N-terminal extension as well as in the insertion (between alpha1 and alpha2) of the archaebacterial S7 stru cture from eubacterial S7 are such that they do not necessitate a major cha nge in the structure of the currently available eubacterial. 16S rRNA. Some of the inserted chains might pass through gaps formed by helices of the 16 S rRNA.