Photoreaction of New Psoralen Analogs with DNA: Sequence and Mutation Specificity in the Escherichia coli lacZ Gene

New thio- and seleno-analogs of psoralen were synthesized and analyzed for their photoreactivity toward DNA. Using oligonucleotides of denned sequence, we first showed that these derivatives predominantly generated interstrand crosslinks at 5′-TpA sites. We also observed a surprisingly high reactivity of 7H-thiopyrano[3,2-f][l]benzofuran-7-one (PSO[0-S]) with the BamHl and PstI oligomers, giving rise to the formation of crosslinks at 5′-ApT sites and of the thymidine-psoralen-cytosine type. Next, the sequence specificity in the photochemical binding of all the compounds was investigated in two DNA fragments encompassing the lacZ gene of Escherichia coli, using the T₄ DNA polymerase sequencing methodology. Resulting maps demonstrated that thio-and seleno-analogs of psoralen preferentially photoreact-ed with thymine and cytosine residues. The AT-rich sequences proved to be particularly reactive sites as did adjacent thymines, especially at C-surrounding residues. Likewise, photoaddition at cytosines in CA/AC context was observed. It was highly significant that all of the derivatives exhibited similar sequence specificities with only minor differences. However, PSO(O-S) differed from the other heteropsoralens. Photoadducts occurred with a higher frequency at AC and CA dinucleotides, and new sites were detected. A comparison with 8-methoxypsor-alen photobinding is also reported. Finally, the mutagenic consequences of photoadducts induced in M13mp19 DNA by PSO(O-S) were determined in a forward system that detects all classes of mutagenic events. The high phototoxicity exhibited by PSO(O-S) could be attributed to crosslinks, and the comparison of the observed mutational specificity with the photoadduct distribution within the same gene showed that mutations were targeted at potential monoadduct sites where photolesions were detected in our footprinting experiments     

Analysis of mutations induced by 4′-hydroxymethyl-4,5′,8-trimethylpsoralen and UVA in Escherichia coli lac Z gene and its regulatory region

The mutagenic consequences of covalent adducts induced in M13mp19 DNA by 4′-hydroxymethyl-4,5′,8-trimethylpsoralen (HMT) and UVA have been determined in a forward mutational system capable of detecting all classes of mutagenic events. The photoreaction mediated by HMT has been carried out at a very low molar ratio of HMT to DNA which favours the induction of cross-links between high affinity reaction sites. When damaged M13mp19 DNA is used to transfect competent Escherichia coli K12 JM105 cells, a five-fold increase in mutation frequency is observed at 3.5% survivors when measured as a loss of β-galactosidase -complementation. The enhanced mutation frequency is largely due to base substitutions, frameshift events and large deletions. The single nucleotide substitutions occur both in the lac Z coding sequence and in its regulatory region. Transversion and transition have been detected with a predominent form consisting of A.T-to-G.C transversion at position + 159. Frameshift mutations have been observed at five positions while three large deletions removing either part of the coding sequence or both the coding and the regulatory regions have been detected with a higher frequency. The spectrum of base substitutions detected between the M13 lac Z⁻ phages surviving to the treatment is totally different from those appearing spontaneously whereas several frameshift events or deletions can already be detected between the spontaneous mutations. Despite the presence of these spontaneous hot spots, the spectrum of mutations recovered after HMT photoaddition appears to be unique and a detailed analysis of the different classes of mutations indicates an important role of cross-links in the production of mutations.     

Complexity for Extended Dynamical Systems

We consider dynamical systems for which the spatial extension plays an important role. For these systems, the notions of attractor, ϵ-entropy and topological entropy per unit time and volume have been introduced previously. In this paper we use the notion of Kolmogorov complexity to introduce, for extended dynamical systems, a notion of complexity per unit time and volume which plays the same role as the metric entropy for classical dynamical systems. We introduce this notion as an almost sure limit on orbits of the system. Moreover we prove a kind of variational principle for this complexity.     

Examples of asymptotic completeness in translation invariant systems with an unbounded number of particles

A tagged particle interacting with a finite but unbounded number of particles is considered. Some examples of asymptotic completeness are given which are uniform in the number of particles.