[The effect of point amino acid substitutions on T4 phage lysozyme stability. II. Transition of a protein molecule to the "molten globule" state with replacements Asp10---His, Asn101---Asp, Arg148---Ser]. / Vliianie tochechnykh aminokislotnykh zamen na stabil'nost' lizotsima faga T4. II. Perekhod belkovoi molekuly v sostoianie "rasplavlennoi globuly" pri zamenakh Asp10---His, Asn101---Asp, Arg148---Ser.

The amino acid replacements (Asp10-->His, Asn101-->Asp, Arg148-->Ser) in the T4 phage lysozyme were obtained by site directed mutagenesis and the plasmid for mutant protein expression was constructed. At acid pH (pH 2.7) the mutant is in the conformational state with properties of the molten globule (Ptitsyn, 1992): 1) the mutant protein molecule is essentially compact, 2) its circular dichroism (CD) spectrum in the near ultra violet (UV) region is drastically reduced in intensity as compared with the wild type protein spectrum, 3) the CD spectrum in the far UV region indicates the presence of a pronounced secondary structure in the mutant, 4) unlike the wild type protein, the mutant protein can bind the hydrophobic fluorescent probe ANS.

[Interaction of EcoRII restriction and modification enzymes with synthetic DNA fragments. IX. Cleavage of substrates with point modifications in the recognition site and flanking sequences]. / Vzaimodeistvie fermentov restriktsii i modifikatsii EcoRII s sinteticheskimi fragmentami DNK. IX. Rasshcheplenie substratov s tochechnymi modifikatsiiami v uchastke uznavaniia i prilegaiushchikh posledovatel'nostiakh.

Ability of the EcoRII restriction endonuclease to cleave 14-base-pair DNA duplexes with nucleotide substitutions in the recognition site CCA/TGG and in the adjacent base pair has been studied. Modifications leading to a local change in the substrate conformation (rU residue in and outside the recognition site, A.A- or A.C-pairs in the flanking sequence) reduce the rate of hydrolysis, the effect being maximal when the modified base pair is outside the recognition site. No digestion occurs when the internal dC-residue of the recognition site is 5-methylated in one or both strands. Replacement of dT residue in the EcoRII recognition site by dfl5U residue results in a dramatic inhibition of hydrolysis. Km and kcat for the cleavage of 14-base-pair DNA duplex have been determined. The cleavage rate of the dT-containing strand of the recognition site in 1.5 fold higher comparing with the dA-containing strand. The cleavage of both strands of the substrate by EcoRII endonuclease is confirmed to proceed in one enzyme-substrate complex.

[DNA-duplexes with phosphoamide bond: interaction with restriction endonucleases EcoRII and SsoII]. / DNK-dupleksy s fosfoamidnoi sviaz'iu: vzaimodeistvie s éndonukleazarestriktsii EcoRII i SsoII.

We studied the interaction of EcoRII and SsoII restriction endonucleases with synthetic DNA duplexes, containing 3'N----5'P and 3'P----5'N phosphoamide internucleotide bonds in one of the cleavage points. Enzymatic hydrolysis of the modified strand of the duplexes is blocked in all cases. The presence of phosphoamide bonds was found to reduce the rate of cleavage of the natural strand by EcoRII and to have no influence in case of SsoII. Properties of the EcoRII endonuclease complex with its substrate, containing non-cleavable 3'N----5'P internucleotide bonds in each cleavage point, were examined. In the presence of Mg2+ ions the equilibrium association constant of the enzyme-substrate complex is 3-fold reduced, and the dissociation rate constant of the complex is increased by 1.5 times.