X-ray analysis of the complex of netropsin with the B-DNA dodecamer of sequence C-G-C-G-A-A-T-TBrC-G-C-G reveals that the antitumor antibiotic binds within the minor groove by displacing the water molecules of the spine of hydration. Netropsin amide NH furnish hydrogen bonds to bridge DNA adenine N-3 and thymine 0-2 atoms occurring on adjacent base pairs and opposite helix strands, exactly as with the spine of hydration. The narrowness of the groove forces the netropsin molecule to sit symmetrically in the center, with its two pyrrole rings slightly non-coplanar so that each ring is parallel to the walls of its respective region of the groove. Drug binding neither unwinds nor elongates the double helix, but it does force open the minor groove by 0.5-2.0 A, and it bends back the helix axis by 8°across the region of attachment. The netropsin molecule has an intrinsic twist that favors insertion into the minor groove of B-DNA, and it is given a small additional twist upon binding. The base specificity that makes netropsin bind preferentially to runs of four or more ACT base pairs is provided not by hydrogen bonding but by close van der Waals contacts between adenine C-2 hydrogens and CH groups on the pyrrole rings of the drug molecule. Substitution of one or more pyrroles by imidazole could permit recognition of G'C base pairs as well, and it could lead to a class of synthetic "lexitropsins," capable of reading any desired short sequence of DNA base pairs.Netropsin and its close relative distamycin ( Fig. 1) are antiviral antitumor antibiotics that, although too toxic for clinical use, have received extensive study as the paradigms of base-specific yet non-intercalative DNA-binding drug molecules. First isolated from Streptomyces netropsis in 1951 (1, 2), netropsin exerts its biological activity by binding tightly to double-helical B-DNA, interfering with both replication and transcription (3, 4). It shows little or no affinity for single-stranded DNA or RNA or for double-stranded RNA or DNA-RNA hybrids (3-5), suggesting that it does not bind to the A helix. It also fails to bind to left-handed Z-DNA; in fact, binding of netropsin to DNA favors A-to-B and Z-to-B helix transitions (6, 7).Chemical protection studies (3,4,8) and Overhauser NMR experiments (9) indicate that netropsin does not intercalate between base pairs, but it binds within the minor groove of the intact double helix, using hydrogen bonds between netropsin amide NH and exposed adenine N-3 and thymine 0-2 on the floor of the minor groove. The drug molecule attaches to clusters of four or more A'T or ITC, but not to G-C, base pairs (3, 10, 11). Alternating A-T-A-T regions bind netropsin less well than continuous runs of A or T (12, 13). Binding involves both an electrostatic component from the two cationic ends and hydrogen bonds from the central three amide NH groups, although neither aspect is absolute- FIG. 1. The netropsin molecule can be regarded as being assembled from (left to right): guanidinium, amide, methylpyrrole, amide, methylpyrr...

Recent advances in DNA synthesis methods have made it possible to carry out single-crystal x-ray analyses of double-stranded DNA molecules of predetermined sequence, with 4 to 12 base pairs. At least one example has been examined from each of the three known families of DNA helix: A, B, and Z. Each family has its own intrinsic restrictions on chain folding and structure. The observed solvent positions in these crystal structures have confirmed earlier fiber and solution measurements, and have led to proposals explaining the transitions from B to A and from B to Z helices. Prospects are improving for an understanding of the mode of bending of DNA in chromatin, and the way in which specific DNA sequences are recognized by drug molecules and repressor proteins.

The DNA double helix is not a regular, featureless barberpole molecule. Different base sequences have their own special signature, in the way that they influence groove width, helical twist, bending, and mechanical rigidity or resistance to bending. These special features probably help other molecules such as repressors to read and recognize one base sequence in preference to another. Single crystal x-ray structure analysis is beginning to show us the various structures possible in the B-DNA family. The DNA decamer C-C-A-A-G-A-T-T-G-G appears to be a better model for mixed-sequence B-DNA than was the earlier C-G-C-G-A-A-T-T-C-G-C-G, which is more akin to regions of poly(dA).poly(dT). The G.A mismatch base pairs at the center of the decamer are in the anti-anti conformation about their bonds from base to sugar, in agreement with nuclear magnetic resonance evidence on this and other sequences, and in contrast to the anti-syn geometry reported for G.A pairs in C-G-C-G-A-A-T-T-A-G-C-G. The ordered spine of hydration seen earlier in the narrow-grooved dodecamer has its counterpart, in this wide-grooved decamer, in two strings of water molecules lining the walls of the minor groove, bridging from purine N3 or pyrimidine O2, to the following sugar O4'. The same strings of hydration are present in the phosphorothioate analog of G-C-G-C-G-C. Unlike the spine, which is broken up by the intrusion of amine groups at guanines, these water strings are found in general, mixed-sequence DNA because they can pass by unimpeded to either side of a guanine N2 amine. The spine and strings are perceived as two extremes of a general pattern of hydration of the minor groove, which probably is the dominant factor in making B-DNA the preferred form at high hydration.

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