Placement of protein and RNA structures into a 5 A-resolution map of the 50S ribosomal subunit

An electron-density map of the large 50S ribosomal subunit from the bacterium Haloarcular marismortui bacterium has been calculated at 5.0 A resolution. Phases were derived from heavy-atom derivatives, intercrystal density averaging and density-modification methods. Over 300 base pairs of A-form RNA duplex were fitted into the map. A polypeptide exit channel was marked by tungsten cluster compounds bound in a heavy-atom-derivatized crystal.