Oxytricha

Kang, C., X. Zhang, R. Ratliff, R. Moyzis, and A. Rich, Crystal structure of four-stranded Oxytricha telomeric DNA. Nature 356 126-131, 1992. 

Macronuclear reorganisation is much more radical in the co-called hy-potrich ciliates, in which fragmentation occurs down to the level of the gene. Finally, each unigenic minichromosome is endowed with the sequences necessary for replication. For example, in Oxytricha, the macronucleus harbours about 24 000 different unlinked genes, each with an average copy number of 950. 

Pluta, A.F., Cani, G.M., Spear, B.B., and Za-kian, V. A. (1984). Elaboration of telomeres in yeast recognition and modification of termini from Oxytricha macronuclear DNA. Proc. 

The first indication that several conformations could be observed came from the comparison of the NMR solution and an earlier X-ray structure of the Oxytricha bimolecular quadruplex. The initial X-ray structure contained many of the features found in the solution structure but differed in the orientation of the thymine loops edge-loops were observed by X-ray as compared to diagonal-loops in the NMR structure. However, a recent analysis corrected this earlier crystallographic study and demonstrated that this quadruplex adopts a structure with two strands forming an antiparallel diagonal arrangement, indicating that the native structure is the same in solution and in the crystalline state. 

Proteins may also modulate the formation of G-quadruplexes. The beta-subunit of the Oxytricha telomere-binding protein acts as a molecular chaperone to... 

F.W. Smith and J. Feigon, Quadruplex structure of Oxytricha telomeric DNA oligonucleotides. Nature, 1992, 356, 164-168. 

S. Haider, G.N. Parkinson and S. Neidle, Crystal structure of the potassium form of an Oxytricha nova G-quadruplex, J. Mol. Biol., 2002, 320, 189-200. 

The X-ray crystal structure at 1.86 A resolution of the Oxytricha nova telomere end binding protein (OnTEBP) in complex with DNA and in the presence of NaCl (PDB ID 1JB7) has also shown that d(G4T4G4) adopts a... 

Structural model of the complex of the 3,6,9-trisubstituted acridine ligand (3) with the intramolecular basket quadruplex structure of the human telomeric sequence, (a) End-stacking with the terminal G-tetradpermits the substituent at the 9-position to interact with the flexible TTA loop structure in a specific manner, (b) X-ray structure of the complex of a di-substituted aninoalkylamido acridine (4) with the dimeric hairpin quadruplex structure derived from the Oxytricha nova sequence d(GGGGTTTTGGGG)2. The drug end-stacks and interacts extensively with the thymine loop ((a) Adapted from ref 25 and (b) from RCSB Protein Data Bank co-ordinates ILIH (ref 49)). 

Subsequently, two X-ray structures have been reported one of a drug-bound dimeric anti-parallel G-quadruplex containing the Oxytricha nova sequence d(GGGGTTTTGGGG) in which a di-substituted aminoalkylamido acridine drug (4, see Figure 2) end-stacks with a terminal G-tetrad and interacts with one of the loops." In the other structure a daunomycin (14) trimer stacks on... 

M.P. Horvath and S.C. Schultz, DNA quartets in a 1.86 A resolution structure of an Oxytricha nova telomeric protein-DNA complex, /. Mol. 

L. Laporte and G.J. Thomas Jr., Structural basis of DNA recognition and mechanism of quadruplex formation by the b subunit of the Oxytricha telomere-binding protein. Biochemistry, 1998, 37, 1327-1335. 

The cocrystal structures cited above provided no evidence of G-quartets or G4 DNA formed within the G-rich telomeric DNA complexed to the protein. While a higher resolution structure (1.86 A) of the Oxytricha telomere endbinding complex identified G-quartets stabilizing antiparallel interactions... 

Does the assumption of the mutator phenotype of the individual cancer cell in a multicellular host constitute a regression to the level of the ancestral unicellular eukaryotes, which were driven by TEs (transposases, retrotransposons, retroviruses), that invaded their genome, and existed there transcribed into mutator DNA sequences The ciliate Oxytricha trifallax possesses a somatic macronucleus and a diploid germline micronucleus [2094]. In the macronucleus reside innumerable... 

---