VISUALIZATION OF RNA CRYSTAL-GROWTH BY ATOMIC-FORCE MICROSCOPY

The crystallization of transfer RNA (tRNA) was investigated using atom ic force microscopy (AFM) over the temperature range from 4 to 16 degr ees C, and this produced the first in situ AFM images of developing nu cleic acid crystals. The growth of the (110) face of hexagonal yeast t RNA(Phe) crystals was observed to occur at steps on vicinal hillocks g enerated by multiple screw dislocation sources in the temperature rang e of 13.5-16 degrees C. Two-dimensional nucleation begins to dominate at 13.5 degrees C, with the appearance of three-dimensional nuclei at 12 degrees C. The changes in growth mechanisms are correlated with var iations in supersaturation which is higher in the low temperature rang e. Growth of tRNA crystals was characterized by a strong anisotropy in the tangential step movement and transformation of growth modes on si ngle crystals were directly observed by AFM over the narrow temperatur e range utilized. Finally, lattice resolution images of the molecular structure of surface layers were recorded. The implications of the str ong temperature dependence of tRNA(Phe) crystal growth are discussed i n view of improving and better controlling crystallization of nucleic acids.