Since the introduction of structural genomics, the protein has been recognized as the most important variable in crystn. Recent strategies to modify a protein to improve crystal quality have included rationally engineered point mutations, truncations, deletions and fusions. Five naturally occurring variants, differing in 1-18 amino acids, of the 177-residue lectin domain of the F17G fimbrial adhesin were expressed and purified in identical ways. For four out of the five variants crystals were obtained, mostly in non-isomorphous space groups, with diffraction limits ranging between 2.4 and 1.1 .ANG. resoln. A comparative anal. of the crystal-packing contacts revealed that the variable amino acids are often involved in lattice contacts and a single amino-acid substitution can suffice to radically change crystal packing. A statistical approach proved reliable to est. the compatibilities of the variant sequences with the obsd. crystal forms. In conclusion, natural variation, universally present within prokaryotic species, is a valuable genetic resource that can be favorably employed to enhance the crystn. success rate with considerably less effort than other strategies. [on SciFinder (R)]