The DNA fragment d(CpGpCpGpCpG) crystallises as a left-handed double helical molecule with Watson-Crick base pairs and an antiparallel organisation of the sugar phosphate chains. The helix has two nucleotides in the asymmetric unit and contains twelve base pairs per turn. It differs significantly from right-handed B-DNA.

A combination of DNA hybridization analyses and tissue sectioning techniques demonstrate that ras gene mutations occur in over a third of human colorectal cancers, that most of the mutations are at codon 12 of the c-Ki-ras gene and that the mutations usually precede the development of malignancy.

No abstract

The anticancer drugs adriamycin and daunomycin have each been crystallized with the DNA sequence d(CGATCG) and the three-dimensional structures of the complexes solved at 1.7- and 1.5-A resolution, respectively. These antitumor drugs have significantly different clinical properties, yet they differ chemically by only the additional hydroxyl at C14 of adriamycin. In these complexes the chromophore is intercalated at the CpG steps at either end of the DNA helix with the amino sugar extended into the minor groove. Solution of the structure of daunomycin bound to d(CGATCG) has made it possible to compare it with the previously reported structure of daunomycin bound to d(CGTACG). Although the two daunomycin complexes are similar, there is an interesting sequence dependence of the binding of the amino sugar to the A-T base pair outside the intercalation site. The complex of daunomycin with d(CGATCG) has tighter binding than the complex with d(CGTACG), leading us to infer a sequence preference in the binding of this anthracycline drug. The structures of daunomycin and adriamycin with d(CGATCG) are very similar. However, there are additional solvent interactions with the adriamycin C14 hydroxyl linking it to the DNA. Surprisingly, under the influence of the altered solvation, there is considerable difference in the conformation of spermine in these two complexes. The observed changes in the overall structures of the ternary complexes amplify the small chemical differences between these two antibiotics and provide a possible explanation for the significantly different clinical activities of these important drugs.

The first step in the replication of the plus-stranded poliovirus RNA is the synthesis of a complementary minus strand. This process is initiated by the covalent attachment of UMP to the terminal protein VPg, yielding VPgpU and VPgpUpU. We have previously shown that these products can be made in vitro in a reaction that requires only synthetic VPg, UTP, poly ( The Picornaviridae family of plus-strand RNA viruses includes a large number of pathogens with widely different host range and disease symptoms (38). At the same time, picornaviruses show a strong similarity in their gene organization and in the mechanism by which they replicate their genomes (58). An unusual feature of their genomes is the presence of a small protein VPg, covalently linked to the 5Ј end of the RNA. Virus replication in the infected host cell is a two-step process, carried out primarily by the viral RNA polymerase, in conjunction with other viral and possibly also cellular proteins. It takes place in small vesicles that are derived from the host's cellular membranes and with which the nonstructural proteins of the virus are associated. First, the incoming viral RNA is transcribed into complementary minus strands which are then used as templates for the synthesis of the progeny plus strands. Although the basic steps of replication are well known, very little is understood about the details of these processes and in particular about the exact functions of the cis-acting RNA structures contained within picornaviral RNAs (1). One of the important unanswered questions about minus-strand synthesis is how the viral RNA polymerase recognizes and selects only its own RNA as template among the many polyadenylated mRNAs that are present in the host cell (45).Poliovirus is perhaps the best known member of the Picornaviridae. Its RNA genome of about 7,500 nucleotides (nt) is composed of a long 5Ј-nontranslated region (NTR), a single open reading frame, a short 3ЈNTR, and a poly(A) tail (Fig. 1A) (32). The 5Ј-terminal UMP of the viral RNA is linked to the hydroxyl group of VPg by a phosphodiester bond (Fig. 1A) (2, 56). The 5ЈNTR consists of two independent domains. The first is a cloverleaf-like structure which is involved both in plus-strand RNA synthesis (3,4,24,70) and in the process of switching from translation to replication (18). The second is a large and complex structure, the internal ribosomal entry site (IRES) (28, 47), which promotes translation of a polyprotein. This polyprotein (Fig. 1A) contains a capsid region (P1) and two nonstructural domains (P2 and P3) (32). The initial cleavage of the polyprotein is carried out by proteinase 2A pro at the P1/P2 site (64). Most other cleavages are mediated by the activities of proteinase 3C pro and its precursor, 3CD pro (22,23,29,73). The proteins of the P2 domain are predominantly involved in inducing the biochemical and structural changes that occur in the infected cell (8), but 2CATPase is also essential for viral genome replication (49). Those of the P3 region are the ones most directly invol...