Gerardo Medina

Mechanism of Pseudomonas aeruginosa RhlR Transcriptional Regulation of the rhlAB Promoter

Pseudomonas aeruginosa contains two transcription regulators (LasR and RhlR) that, when complexed with their specific autoinducers (3-oxo-dodecanoyl-homoserine lactone and butanoyl-homoserine lactone, respectively) activate transcription of different virulence-associated traits. We studied the RhlR-dependent transcriptional regulation of the rhlAB operon encoding rhamnosyltransferase 1, an enzyme involved in the synthesis of the surfactant monorhamnolipid, and showed that RhlR binds to a specific sequence in the rhlAB regulatory region, both in the presence and in the absence of its autoinducer. Our data suggest that in the former case it activates transcription, whereas in the latter it acts as a transcriptional repressor of this promoter. RhlR seems to repress the transcription of other quorum-sensing-regulated genes; thus, RhlR repressor activity might be of importance in the finely regulated expression of P. aeruginosa virulence-associated traits.

The Pseudomonas aeruginosa rhlAB Operon Is Not Expressed during the Logarithmic Phase of Growth Even in the Presence of Its Activator RhlR and the Autoinducer N-Butyryl-Homoserine Lactone

The Pseudomonas aeruginosa rhlAB operon encodes the enzyme rhamnosyltransferase 1, which produces the biosurfactant mono-rhamnolipid; rhlAB induction is dependent on the quorum-sensing transcription activator RhlR complexed with the autoinducer N-butyryl-homoserine lactone (C(4)-HSL). In this work we studied rhlAB induction in a P. aeruginosa and Escherichia coli background. We found that, in both bacteria, its expression is not induced during the logarithmic phase of growth even in the presence of RhlR and C(4)-HSL. Additionally, we found that rhlAB expression is partially sigma(s) dependent.

CU-O STRETCHING FREQUENCY CORRELATION WITH PHENANTHROLINE PKA VALUES IN MIXED COPPER COMPLEXES

Abstract A study of Cu–O vibration frequencies in 32 isostructural mixed copper complexes containing eight differently substituted phenanthrolines and four different oxygen donor bidentate ligands, is presented. In all cases linear correlation was found between phenanthroline p K a values and the studied vibration frequencies, suggesting that an increase in phenanthroline basicity weakens Cu–O bonds in this type of compound.

Expression of Cholera Toxin under Non-AKI Conditions in Vibrio cholerae El Tor Induced by Increasing the Exposed Surface of Cultures

The regulatory systems controlling expression of the ctxAB genes encoding cholera toxin (CT) in the classical and El Tor biotypes of pathogenic Vibrio cholerae have been characterized and found to be almost identical. Notwithstanding this, special in vitro conditions, called AKI conditions, are required for El Tor bacteria to produce CT. The AKI conditions involve biphasic cultures. In phase 1 the organism is grown in a still tube for 4 h. In phase 2 the medium is poured into a flask to continue growth with shaking. Virtually no expression of CT occurs if this protocol is not followed. Here we demonstrated that CT expression takes place in single-phase still cultures if the volume-to-surface-area ratio is decreased, both under air and under an inert atmosphere. The expression of key genes involved in the regulation of CT production was analyzed, and we found that the expression pattern closely resembles the in vivo expression pattern.

A ferrimagnetic cyclic tetranuclear copper(II) complex: cyclo-[tetrakis(μ-η3-hydroxyethanoato-1κO:2κ2O′,O″)tetrakis(1,10-phenanthroline)tetracopper] tetranitrate dihydrate. Structural and magnetic properties

The crystal structure and magnetic analysis of a cyclic tetranuclear copper(II) complex with formula [Cu4(phen)4(glyOH)4](NO3)4·2H2O, where phen = 1,10-phenanthroline and glyOH = glycolate are described. Copper atoms are connected through bridging carboxylate groups from chelated glycolate units, forming a butterfly-type framework. Magnetic measurements as a function of temperature (2–300 K) and at 10, 100 and 1000 G were performed. The behaviour of the T–T curves is characteristic of ferrimagnetic-type coupling. Several theoretical magnetic models were tested to study the experimental data. A tetranuclear model of S = 1/2 coupled spins in a C2v symmetry, is the one that better describes the magnetic response of the solid product over the whole range of experimental conditions explored. This model allows us to suggest the existence of a very weak antiferromagnetic coupling for two different pairs of Cu(II) centers, and a weak ferromagnetic coupling between those pairs. Besides this, negligible magnetic coupling at the mean field level is proposed. It is important to point out that although the system is homonuclear, with coordination spheres which are chemically equivalent for all the metallic centers, and which shows (at room temperature) equivalent internuclear distances between first neighbouring copper atoms, different types of intramolecular magnetic coupling are inferred. To our knowledge, the complex here studied appears to be the first case of a cyclic tetranuclear Cu(II) system showing global ferrimagnetic coupling.

INFRARED ASSIGNMENT OF BIS(GLYCOLATO)-BIS(PYRIDINE) METAL(II) COMPOUNDS AND CRYSTAL STRUCTURE OF TRANS-BIS(GLYCOLATO)-CIS-BIS(PYRIDINE)NICKEL(II) DIHYDRATE

Abstract Infrared spectra of the coordination compounds [MG2(py)2], M(II)=Co, Ni, Cu and Zn; G=glycolato, py=pyridine, have been fully assigned by means of py and py-d 5 and glycolato α—OH and α—OD (G-d) labelling as well as metal ion substitution in the 4000–70cm−1 region. The crystal structure of the Ni(II) compound is presented and the spectra of the compounds are discussed on the basis of their structure and their bonding to the glycolato and pyridine ligands. Vibrational frequencies obtained for the Ni(II) compound are compared to those obtained by calculations carried out using the Gaussian 94 program package.