Gene expression from multicopy T7 promoter vectors proceeds at single copy rates in the absence of T7 RNA polymerase

Three different genes (trpR+, tyrR+ and phi (trpR-lacZ)) were inserted into pET3a, a multicopy transcription-translation vector designed by Rosenberg et al. (1) for the T7 RNA polymerase-driven overexpression of proteins in Escherichia coli. Gene orientation was in the anticlockwise ("silent") direction. Gene expression in the absence of T7 RNA polymerase was evaluated either directly using lacZ reporter systems or indirectly by observing the susceptibility of plasmid-bearing tester strains to inhibition by an aromatic amino acid analog. The production of repressor proteins and of a Trp repressor-LacZ chimera was readily detected, at levels comparable to those of haploid trpR+ or tyrR+ E. coli strains. Such T7 vector constructs thus have two especially useful properties: first, they provide a means for the high-level production of various proteins in E. coli; second, they offer a technically advantageous point of departure for structure-function studies of genes whose overexpression from multicopy plasmids would normally be cytotoxic.

Properties of the nucleic acid photoaffinity labeling agent 3-azidoamsacrine

This paper reports the study of the photochemical, physical, and biological properties of 3-azidoamsacrine. The binding of 3-azidoamsacrine to DNA was studied with UV spectroscopy. The UV spectral behavior is quite similar to that of the parent amsacrine and argues that 3-azidoamsacrine is a good photoaffinity labeling agent for amsacrine. The biological properties (cytotoxicity and mutagenicity) of 3-azidoamsacrine in the mammalian mutagenesis V79 and L5178Y assay systems were measured. Light-activated 3-azidoamsacrine is toxic, but not mutagenic, to V79 cells. 3-Azidoamsacrine with and without light activation, as well as amsacrine, are toxic and mutagenic to L5178Y cells. To probe the interactions of 3-azidoamsacrine with DNA, studies of the photoreactivity of this compound were conducted. 3-Azidoamsacrine was photolyzed in the presence of the plasmid pBR322, and the effect of the photoadducts on restriction endonuclease cleavage was investigated. Amsacrine and 3-azidoamsacrine, without light activation, did not block any of the restriction endonucleases. Light-activated 3-azidoamsacrine blocked cleavage by the restriction endonucleases AluI, HinfI, NciI, NaeI, DraI, Sau96I, HpaII, and HaeIII. Photolysis experiments with mononucleosides, blocked mononucleosides, dinucleotides, and DNA all indicated that 3-azidoamsacrine formed adducts with G and A. The structures of these adducts are discussed based upon mass spectral data. Thus, it appears that 3-azidoamsacrine covalently attaches to DNA and that this covalent binding results in the production of toxic and, in some cases, mutagenic lesions in mammalian cells and the inhibition of restriction endonuclease cleavage of DNA.

Solution conformation of the thermolysin inhibitors carbobenzoxy-L-phenylalanine and beta-phenylpropionyl-L-phenylalanine and comparison of the solution conformation to the enzyme-bound conformation

The conformations of enzyme inhibitors in solution and bound to the enzyme thermolysin are investigated as a convenient model for understanding the relationship between the conformation of drugs in solution and at the receptor. The solution conformations of carbobenzoxy-L-phenylalanine (I) and beta-phenylpropionyl-L-phenylalanine (II) are determined using nuclear magnetic resonance spectroscopy. These studies show that I and II have essentially the same conformation about both the alpha-beta bond and the C alpha-N bond in Me2SO-d6, acetone-d6, methanol-d4, chloroform-d, and D2O. In addition, the conformations of I and II are similar to phenylalanine and other phenylalanine derivatives. Comparison of the lowest energy solution conformer to that bound by the enzyme thermolysin shows that the lowest energy conformer (in solution) of carbobenzoxy-L-phenylalanine is bound by thermolysin; on the other hand, it is the highest energy conformer (in solution) of beta-phenylpropionyl-L-phenylalanine which is bound to the enzyme. This, to our knowledge, is one of the first experimental conformations of the hypothesis that an enzyme does not always bind the lowest energy conformer of an inhibitor.

The biosynthesis of the antibiotic pyrrolnitrin by Pseudomonas aureofaciens

Feeding experiments with tryptophan samples labeled specifically with radioactive and stable isotopes have shown that Pseudomonas aureofaciens converts this amino acid into pyrrolnitrin in such a way that the indole nitrogen gives rise to the nitro group, the amino group becomes the pyrrole nitrogen, C-3 of the precursor side chain becomes C-3 of the antibiotic, and H-2 of the indole ring and H-alpha of the side chain give rise to H-5 and H-2 of pyrrolnitrin, respectively. Only the L-isomer of tryptophan is incorporated with retention of the alpha-hydrogen and the amino nitrogen. From the D-isomer the labels from these two positions are lost. The obvious conclusion that L-tryptophan is the more immediate precursor is, however, contradicted by the better incorporation of D- than L-tryptophan into the antibiotic. Several potential pathway intermediates were evaluated for incorporation and 4-(0-aminophenyl)-pyrrole was found to be a good precursor. The results area discussed in terms of a plausible pathway for pyrrolnitrin biosynthesis.

Comparison of solution conformational preferences for the hallucinogens bufotenin and psilocin using 360-MHz proton NMR spectroscopy

The 360-MHz 1H NMR spectra of bufotenin and psilocin were obtained, both as the free bases in CDCl3 and as protonated salts in D2O. Coupling constants for the side-chain methylenes were derived using the LAOCN3 program. These time-averaged coupling constants indicate that the trans and gauche rotamers of both compounds have about equal energy in D2O. There is a slight excess of the trans rotamer of bufotenin in CDCl3. For psilocin, in contrast, the gauche form is highly favored in CDCl3. The magnitude of this stabilization was estimated at about 1 kcal/mol using rotamer populations and free energy of transfer from published partitioning studies. It is suggested that this could result from a very weak hydrogen bond. On the other hand, the difference in partitioning between bufotenin and psilocin, which seems to be a major determinant of biological activity, is largely due to a difference in the basicity of the two compounds. The pKa values for the amino group of psilocin and bufotenin were determined to be 8.47 and 9.67, respectively.

Model studies of pyridoxal Schiff's bases. Coplanarity and intramolecular hydrogen bonding

The interactions between the pi cloud of the aromatic ring and the pi-electron pair of the imine double bond of aromatic oximes as model compounds of pyridoxal Schiff's bases have been studied by high-resolution carbon-13 magnetic resonance spectroscopy. The coplanarity and intramolecular hydrogen bonding have been determined by 13C-1H long range couplings. This detailed investigation of 13C-1H coupling also provides unambiguous proof of the existence of the "enol-imine" tautomers in chloroform and dimethyl sulfoxide solutions. The tautomerism between the "enol-imine" and "keto-enamine" is discussed.