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Our dimethyl sulfate modification experiments suggest that (dG). stretches within single-stranded DNA fragments, which represent the simplest model for telomeric sequences, adopt a complex intrastrand structure other than a simple hairpin. We present a molecular model for the DNA structure that conforms to dimethyl sulfate methylation data. The principal element of this G-DNA structure is a quadruple helix formed by pairwise antiparallel segments of the twicefolded (dG). stretch. This quadruple core has two wide and two narrow grooves connected by three loop-shaped segments. The strong stacking interactions of the neighboring guanine tetrads and the large number of hydrogen bonds formed can be the primary reasons that such structures are favored over a common hairpin for long (dG pPG27, which contain (dG),-(dC)" inserts at the Pst I site of pUC18 with n = 37 and 27, respectively. It has been found (11) that, after the short insert-containing restriction fragments have been denatured, the (dG),-and (dC)Q-containing strands renature much more slowly than do those of arbitrary sequence. In addition, the (dG)n strand has an abnormally high mobility in polyacrylamide gel electrophoresis (PAGE) (11). These data argue in favor of the existence within (dG),-containing single-stranded fragments of an intrastrand structure (G structure) stabilized by formation of hydrogen bonds between guanines. In this paper we describe the results of methylation experiments on the oligo(dG)-containing frag- (14). However, the G-structure methylation pattern for longer stretches-(dG)27 and (dG)37-is more complicated (Fig. 1). Clear maxima and minima of modification are seen against the background of general diminution of peak amplitude associated with overmodification (more than one dimethyl sulfate modification per fragment). The maxima between extremities are positioned at one-quarter, one-half, and three-quarters of the way along the insert. This suggests that the (dG), strand is bent in three places, with three exposed and methylated loops. The fact that all four segments flanking the loops are simultaneously protected against methylation at their N7 positions can only mean (15) that these loci are shielded by formation of Hoogsteen-like hydrogen-bonded tetrads (16,17) like that shown in Fig. 2. But in any event, the similarity of disposition of maxima and minima for the (dG)27 and (dG)37 oligomers shows that these two share a common structure, whatever §Present address:

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The efficiency and specificity of RNA-protein cross-linking in the 30s subunit of Escherichia coli ribosomes, induced by low-intensity (lo1' photons cm-' s-l, 254 nm) and high-intensity [(1.6-6.8) x photons cm-' s-', 266 nm, pulse duration lo-* s] ultraviolet radiation, are studied. Under the former conditions proteins S4, S7 and S9/Sll, and under the latter conditions these proteins together with S3, S18 and S20, are cross-linked to 16s RNA. Biphotonic processes operate in the latter case. In the presence of 2-mercaptoethanol cross-linking occurs either directly, via a higher excited state or via activated intermediates with life-times less than 25 ns. Crosslinks thus formed are specific, i.e. they are formed between regions of macromolecules which are in contact in the native (non-disturbed) complex prior to excitation. The efficiency of cross-linking (per photon absorbed) is 20 -100 times higher upon two-step excitation as compared with single-step excitation and an analysable number of cross-links are produced in a single pulse. Only base U-1239 of 16s RNA is cross-linked to protein S7 by lowintensity radiation, whereas the adjacent base, G-1240 is also involved in laser-induced cross-linking. A transition from the former to the latter conditions allows one to reduce the duration of irradiation from several minutes to several nanoseconds.The formation of functional nucleoproteins depends on interactions between regions of polynucleotides and proteins with specific primary and higher structures. The stability of such complexes is determined by the cooperativity of multiple local non-covalent interactions, in particular between nucleic acid bases and amino acid residues, which can occur only between spatially adjacent groups with suitable mutual orientation and electronic structure.The identification of directly neighbouring macromolecules (i. e. those in contact) and the localisation, within their primary and higher structure, of neighbouring fragments and residues produces information about the functional topography of both component macromolecules and the complex as a whole. This can be achieved by generation of covalent bonds between components of macromolecules which are in direct contact within the original complex.The ultraviolet radiation of standard light sources is widely used for generation of polynucleotide-protein crosslinks in nucleoproteins [l, 21. In these cases reactions take place via lower excited states of nucleic bases [3]. Photochemical conversions via higher excited states populated by high-intensity (laser) irradiation are more efficient than via lower excited states [4 -81. Therefore transition from standard to laser light sources could result in the increase of cross-linking efficiency and in formation of additional crosslinks, which are unable to arise at all via lower excited states.Correspondence to E. 1. Budowsky, Institut Organicheskoj Khimii imeni N. D. Zelinskogo, Akademiya Nauk SSSR, Leninskij prospekt 47, Moskva, USSR 117913Some preliminary results of this study were prese...