Psoralen-sensitive mutant pso9-1 of Saccharomyces cerevisiae contains a mutant allele of the DNA damage checkpoint gene MEC3.

Complementation analysis of the pso9-1 yeast mutant strain sensitive to photoactivated mono- and bifunctional psoralens, UV-light 254 nm, and nitrosoguanidine, with pso1 to pso8 mutants, confirmed that it contains a novel pso mutation. Molecular cloning via the reverse genetics complementation approach using a yeast genomic library suggested pso9-1 to be a mutant allele of the DNA damage checkpoint control gene MEC3. Non-complementation of several sensitivity phenotypes in pso9-1/mec3Delta diploids confirmed allelism. The pso9-1 mutant allele contains a -1 frameshift mutation (deletion of one A) at nucleotide position 802 (802delA), resulting in nine different amino acid residues from that point and a premature termination. This mutation affected the binding properties of Pso9-1p, abolishing its interactions with both Rad17p and Ddc1p. Further interaction assays employing mec3 constructions lacking the last 25 and 75 amino acid carboxyl termini were also not able to maintain stable interactions. Moreover, the pso9-1 mutant strain could no longer sense DNA damage since it continued in the cell cycle after 8-MOP + UVA treatment. Taken together, these observations allowed us to propose a model for checkpoint activation generated by photo-induced adducts.

Kinetic characterization of extracellular alpha-amylase from a derepressed mutant of Bacillus licheniformis.

Three strains of Bacillus licheniformis were isolated and screened for alpha-amylase production by solid-state fermentation. Of these, IS-2 gave relatively higher enzyme production (32+/-2.3 U/[g.min]) and was selected for improvement after treatment with N-methylnN-nitro N-nitroso guanidine (NG) or nitrous acid (NA) to enhance its hydrolytic potential. Among the mutant variants, NA-14 gave higher enzyme production (98+/-1.6 U/[g.min]), and hence, was selected for kinetic and thermal characterization. M1 as a moistening agent (pH 7.0, optimized) supported 2.65-fold improved amylolytic activity by the derepressed mutant 72 h after inoculation. The values of product yield coefficient (Y p/x = 1833.3 U/g) and specific rate constant (qp = 25.46 U/[g.h]) with starch were severalfold improved over those from other carbon sources and the other cultures. The purified enzyme from NA-14 was most active at 40 degrees C; however, the activity remained almost constant up to 44 degrees C. The NA-induced random mutagenesis substantially improved the enthalpy (DeltaH D = 94.5+/-11 kJ/mol) and entropy of activation (DeltaS = -284+/-22J/[mol.K]) for alpha-amylase activity and substrate binding for starch hydrolysis. The results of this study (117.8+/-5.5 U/[g.min]) revealed a concomitant improvement in the endogenous metabolism of the mutant culture for alpha-amylase production.

Ab initio and DFT studies on nitrosoguanidine tautomers.

Isolated nitrosoguanidine tautomers have been subjected to 6-31G(d,p), 6-31G(d,p) /(MP2), B3LYP/6-311G(d,p) and B3LYP/6-311++G(d,p) type quantum chemical analyses in the gas phase. The geometrical features and energetics of some conformers of the tautomers are reported. The nitrosimine form has the highest stability than the others and the diazoic acid form is the least stable one. The nitrosoimine form has the highest HOMO and LUMO energies. Whereas, the nitrosamine form possesses the lowest HOMO and the diazoic acid form has the lowest LUMO energies.

Structural determinants in domain II of human glutathione transferase M2-2 govern the characteristic activities with aminochrome, 2-cyano-1,3-dimethyl-1-nitrosoguanidine, and 1,2-dichloro-4-nitrobenzene.

Two human Mu class glutathione transferases, hGST M1-1 and hGST M2-2, with high sequence identity (84%) exhibit a 100-fold difference in activities with the substrates aminochrome, 2-cyano-1,3-dimethyl-1-nitrosoguanidine (cyanoDMNG), and 1,2-dichloro-4-nitrobenzene (DCNB), hGST M2-2 being more efficient. A sequence alignment with the rat Mu class GST M3-3, an enzyme also showing high activities with aminochrome and DCNB, demonstrated an identical structural cluster of residues 164-168 in the alpha6-helices of rGST M3-3 and hGST M2-2, a motif unique among known sequences of human, rat, and mouse Mu class GSTs. A putative electrostatic network Arg107-Asp161-Arg165-Glu164(-Gln167) was identified based on the published three-dimensional structure of hGST M2-2. Corresponding variant residues of hGSTM1-1 (Leu165, Asp164, and Arg167) as well as the active site residue Ser209 were targeted for point mutations, introducing hGST M2-2 residues to the framework of hGST M1-1, to improve the activities with substrates characteristic of hGST M2-2. In addition, chimeric enzymes composed of hGST M1-1 and hGST M2-2 sequences were analyzed. The activity with 1-chloro-2,4-dinitrobenzene (CDNB) was retained in all mutant enzymes, proving that they were catalytically competent, but none of the point mutations improved the activities with hGST M2-2 characteristic substrates. The chimeric enzymes showed that the structural determinants of these activities reside in domain II and that residue Arg165 in hGST M2-2 appears to be important for the reactions with cyanoDMNG and DCNB. A mutant, which contained all the hGST M2-2 residues of the putative electrostatic network, was still lacking one order of magnitude of the activities with the characteristic substrates of wild-type hGST M2-2. It was concluded that a limited set of point mutations is not sufficient, but that indirect secondary structural affects also contribute to the hGST M2-2 characteristic activities with aminochrome, cyanoDMNG, and DCNB.