Nitrocellulose filter binding technique has been used to study the binding of E. coli RNA polymerase to synthetic DNA duplexes (100-200 base pairs), containing the repeating fragments of promoters. It has been shown, that the duplex, containing the repeats of "ideal". Pribnow box forms heparin resistant complexes with enzyme, the stability of which is comparable with that of lacUV5 promoter complexes (the half life is approximately 200 min). The synthetic polynucleotide with repeating trp-promoter-operator sequence less stable complexes with RNA polymerase, the half life of which being 30 min.