[Effect of N4-methylcytosine and 5-methylcytosine on the stability of the DNA helix]. / Vliianie N4-metiltsitozina i 5-metiltsitozina na stabil'nost' spirali DNK.

The thermodynamic parameters (delta H, delta S) of the helix-coil transition of self-complementary oligonucleotides d(CGCGCGCG), d(CG5mCGCGCG), d(CG4mCGCGCG), d(GGACCCGGGTCC), d(GGA5mCCCGGGTCC), and d(GGA4mCCCGGGTCC) were determined. The substitution of 4mC for C was found to decrease the melting temperature of the oligonucleotides. The destabilization effect of the two substitutions is equivalent to the change of A.T for G.C pair. The free energy decrease of the helix-coil transition due to the introduction of two 4mC into an octanucleotide was estimated to be 1,24 kcal/mol.

[Specificity of action of DNA methylases BcnI, CfrI and Cfr10I]. / Spetsifichnost' deistviia DNK-metilaz BcnI, CfrI i Cfr10I.

The site specificity of three DNA methylases BcnI, CfrI and Cfr10I was determined to be 5'Cm4C(C/G)GG, 5'PyGGm5CCPu and 5'Pum5CCGGPy, respectively. Using the modification methylases under investigation with known restriction endonucleases, fourteen new DNA cleavage specificities can be created. Some aspects of the use of restriction endonucleases in DNA methylation analysis are discussed.

[Specificity of new restrictases and methylases. Unusual modification of cytosine at position 4]. / Izuchenie spetsifichnosti novykh restriktaz i metilaz. Neobychnaia modifikatsiia tsitozina po 4-mu polozheniiu.

Fourteen restriction endonucleases and 4 methylases were isolated and purified from 14 strains of Citrobacter freundii and Escherichia coli, which were isolated from natural sources. To determine the nucleotide sequence recognized by the endonucleases a comparison of DNA cleavage patterns, the evaluation of the cleavage frequency of some DNA with known recognition sequences and mapping was used. It was determined that Cfr101 is a new enzyme recognizing 5'PuCCGGPy. Other restriction enzymes isolated were isoschizomers of: Cfr5I, Cfr11I, Eco60I, Eco61I--EcoRII; Cfr4I, Cfr8I, Cfr13I--Sau96I; Cfr6I--PvuII, Cfr9I--SmaI, Eco26I--HgiJII; Eco32I--EcoRV; Eco52I--XmaIII; Eco56I--NaeI. Some of the enzymes in C. freundii and E. coli were found for the first time. The methylases MCfrI; MCfr6I, MCfr9I and MCfr10I recognize the same nucleotide sequence as specific endonucleases isolated from the same strain. DNA modification in vitro by MCfrI and MCfr10I yields 5-methylcytosine and 4-methylcytosine by MCfr6I and MCfr9I.