Conserved nucleotides in an RNA essential for hepatitis B virus replication show distinct mobility patterns.

The number of regulatory RNAs with identified non-canonical structures is increasing, and structural transitions often play a role in their biological function. This stimulates interest in internal motions of RNA, which can underlie structural transitions. Heteronuclear NMR relaxation measurements, which are commonly used to study internal motion, only report on local motions of few sites within the molecule. Here we have studied a 27-nt segment of the human hepatitis B virus (HBV) pregenomic RNA, which is essential for viral replication. We combined heteronuclear relaxation with the new off-resonance ROESY technique, which reports on internal motions of H,H contacts. Using off-resonance ROESY, we could for the first time detect motion of through-space H,H contacts, such as in intra-residue base-ribose contacts or inter-nucleotide contacts, both essential for NMR structure determination. Motions in non-canonical structure elements were found primarily on the sub-nanosecond timescale. Different patterns of mobility were observed among several mobile nucleotides. The most mobile nucleotides are highly conserved among different HBV strains, suggesting that their mobility patterns may be necessary for the RNA's biological function.

Solution structure of the apical stem-loop of the human hepatitis B virus encapsidation signal.

Hepatitis B virus (HBV) replication is initiated by HBV RT binding to the highly conserved encapsidation signal, epsilon, at the 5' end of the RNA pregenome. Epsilon contains an apical stem-loop, whose residues are either totally conserved or show rare non-disruptive mutations. Here we present the structure of the apical stem-loop based on NOE, RDC and (1)H chemical shift NMR data. The (1)H chemical shifts proved to be crucial to define the loop conformation. The loop sequence 5'-CUGUGC-3' folds into a UGU triloop with a CG closing base pair and a bulged out C and hence forms a pseudo-triloop, a proposed protein recognition motif. In the UGU loop conformations most consistent with experimental data, the guanine nucleobase is located on the minor groove face and the two uracil bases on the major groove face. The underlying helix is disrupted by a conserved non-paired U bulge. This U bulge adopts multiple conformations, with the nucleobase being located either in the major groove or partially intercalated in the helix from the minor groove side, and bends the helical stem. The pseudo-triloop motif, together with the U bulge, may represent important anchor points for the initial recognition of epsilon by the viral RT.

The apical stem-loop of the hepatitis B virus encapsidation signal folds into a stable tri-loop with two underlying pyrimidine bulges.

Reverse transcription of hepatitis B virus (HBV) pregenomic RNA is essential for virus replication. In the first step of this process, HBV reverse transcriptase binds to the highly conserved encapsidation signal, epsilon (epsilon), situated near the 5' end of the pregenome. epsilon has been predicted to form a bulged stem-loop with the apical stem capped by a hexa- loop. After the initial binding to this apical stem- loop, the reverse transcriptase synthesizes a 4 nt primer using the bulge as a template. Here we present mutational and structural data from NMR on the apical stem-loop of epsilon. Application of new isotope-labeling techniques (13C/15N/2H-U-labeling) allowed resolution of many resonance overlaps and an extensive structural data set could be derived. The NMR data show that, instead of the predicted hexa-loop, the apical stem is capped by a stable UGU tri-loop closed by a C-G base pair, followed by a bulged out C. The apical stem contains therefore two unpaired pyrimidines (C1882 and U1889), rather than one as was predicted, spaced by 6 nt. C1882, the 3' neighbour to the G of the loop-closing C-G base pair, is completely bulged out, while U1889 is at least partially intercalated into the stem. Analysis of 205 of our own HBV sequences and 1026 strains from the literature, covering all genotypes, reveals a high degree of conservation of epsilon. In particular, the residues essential for this fold are either totally conserved or show rare non-disruptive mutations. These data strongly indicate that this fold is essential for recognition by the reverse transcriptase.

Structure elucidation of the hepatitis B virus encapsidation signal by NMR on selectively labeled RNAs.

Hepatitis B virus (HBV) HBV is DNA virus with a unique replication strategy, which involves reverse transcription of its pregenomic RNA. Essential for this reverse transcription are the 5'- and 3'-ends of its pregenomic RNA (5'-RT-RNA and 3'-RT-RNA, respectively) which form conserved bulged stem-loop structures. The 5'-RT-RNA consists of a 67 nucleotide bulged stem-loop structure, epsilon, which constitutes the signal for encapsidation of the pregenomic RNA and subsequent reverse transcription. The reverse transcriptase (RT) initially binds to the completely conserved apical loop of epsilon and a 4-nucleotide primer is synthesized from the adjacent 6-nucleotide bulge. Structural studies of epsilon can provide important parameters required for the design of RNA targeted anti- viral drugs directed against Hepatitis B virus. NMR studies of large RNA systems (> ca. 50 nucleotides) require novel approaches, e.g., different labeling schemes and reduction of the system into separate structural building blocks. Recently, a new method of synthesizing (13)C/(15)N/(2)H labeled nucleotides has been developed based on converting specifically labeled glucose and bases into nucleotides by using enzymes from the pentose phosphate pathway and nucleotide and salvage pathways. These NTPs give a large freedom in designing different labeling patterns in in vitro synthesized RNAs under study for NMR. This opens up the way for NMR studies of RNAs that are considerably above the present size limit (up to 150 nucleotides). Here this new technique is applied for structural studies on 27, 36 and 61 nucleotides long RNA fragments, mimicking different regions of epsilon.