Crystallization of the dimerization-initiation site of genomic HIV-1 RNA: preliminary crystallographic results

The genomic RNA of all retroviruses is encapsidated in virions as a dimer of single-stranded chains held together near their 5'-­end. For HIV-1, the initial site of dimerization has been shown to be a hairpin with a nine-residue loop containing a self-complementary sequence of six residues. This structure is proposed to promote dimerization by loop-loop interaction and formation of a so-called `kissing complex'. A 23-nucleotide RNA strand containing the loop enclosed by a seven base-pair stem has been synthesized. This oligomer was crystallized by the vapour-diffusion method at 310 K, pH 6.5, with methyl-pentanediol as the precipitant agent in the presence of MgCl₂, KCl and spermine. Quasi-complete diffraction data were obtained at 2.7 Å resolution with a conventional X-ray source and at 2.3 Å resolution on a synchrotron beamline. The space group is P3₁21 or its enantiomorph P3₂21, with cell parameters a = b = 60.1, c = 65.9 Å at ambient temperature, or a = b = 59.0, c = 64.3 Å in a nitrogen-gas stream. There are two oligomers per asymmetric unit as determined from absorbance measurements of a dissolved crystal whose volume was carefully determined. In some cases, either perfectly or partially twinned crystals were obtained. Perfect twinning is detected by an apparent hexagonal symmetry and yields unusable crystallographic data, whilst partial twinning yields usable data after adequate processing. Structure solution is under way by searching for heavy-atom derivatives and systematically substituting bromo- or iodo-uridines for uridines.