TectoRNA and 'kissing-loop' RNA: atomic force microscopy of self-assembling RNA structures.

RNA molecules have been much less studied by atomic force microscopy (AFM) than have DNA molecules. In this paper, AFM imaging is presented for two different RNA molecules able to self-assemble into complex supramolecular architectures. The first one is a molecular dimer of a 230-nt RNA fragment coming from the RNA genome of a murine leukaemia virus. The monomeric RNA fragment, which appears by AFM as an elongated structure with a mean aspect ratio of 1.4, assembles into a dimer of elongated structures through the formation of a 'kissing-loop' RNA interaction. The second one is a large supramolecular fibre formed of artificial self-assembling RNA molecular units called tectoRNA. The fibre lengths by AFM suggest that there are 50-70 tectoRNA units per fibre. Some methods and limitations are presented for measuring molecular volumes from AFM images.