Guanine-rich oligonucleotides targeted to a critical R . Y site located in the Ki-ras promoter. The effect of competing self-structures on triplex formation.
The promoter of the murine Ki-ras proto-oncogene contains a (C+G)-rich homopurine . homopyrimidine (R . Y) sequence that is essential for transcription activity. We have designed two G-rich oligonucleotides, d(TGGGTGGGTGGTTGGGTGGG) (20GT) and d(AGGGAGGGAGGAAGGGAGGG) (20AG), that have the potential to bind the critical Ki-ras sequence via triplex-helix formation. Band-shift experiments have shown that 20GT binds the Ki-ras R . Y duplex with a delta G value of -40 +/- 5 kJ/mol, while 20AG appeared to have a lower affinity under the experimental conditions adopted: 50 mM Tris/HCl, pH 7.4, 50 mM NaCl, 5 mM MgCl2, 25 degrees C. In the absence of Mg2+, 20GT did not bind to the Ki-ras R . Y target, while 20AG exhibited the same affinity observed in the magnesium-containing buffer. To gain insight into the solution properties of 20GT and 20AG, we have performed several experiments including polyacrylamide gel electrophoresis (PAGE), hydroxyapatite chromatography, ultraviolet absorption melting and circular dichroism (CD). We found that 20AG rapidly self-associates into presumably a duplex, even at low concentration (< 1 microM), while 20GT forms aggregates slowly, a process favoured by high oligonucleotide concentrations (> 25 microM). The critical Ki-ras sequence was inserted in Bluescript KS+, downstream from the T7 promoter, to investigate to what extent 20AG and 20GT, which are directed against the R . Y target, are able to inhibit T7 RNA polymerase transcription, under near-physiological conditions. Transcription experiments conducted in vitro at pH 7.4 have shown that oligonucleotide 20GT produced a remarkable repression of T7 RNA polymerase activity in the concentration range (10-25 microM), whereas 20AG had little effect on transcription. In conclusion, the results of this work together with other data reported in the literature [Olivas, W. M. & Maher, L. J. III (1995) Biochemistry 34, 278-284; Noonberg, S. B., François, J.-C., Garestier, T. & Hélène, C. (1995) Nucleic Acids Res. 23, 1956-1963], demonstrate that G-rich oligonucleotides, in particular (G,A)-sequences, may raise problems for in vivo application due to self-aggregation.