On/off regulation of gene expression at the level of splicing

The past two years have seen the discovery of three independent cases in which expression of a eukaryotic protein gene product is turned on and off by controlling splicing events necessary to produce the corresponding mRNA. Various considerations suggest that such on/off regulation at the level of splicing may be unexpectedly common.     

supG and supL in Escherichia coli code for mutant lysine tRNAs+.

We have determined the nucleotide sequences of lysine tRNAs isolated from strains containing one or the other of two Escherichia coli ochre suppressors, supG and supL. Each strain, besides producing wild-type lysine tRNA, has a mutant lysine tRNA species that apparently can read the polypeptide chain termination codons UAA and UAG. The mutant tRNAs from supG and supL strains are identical. In each case the suppressor tRNA has an A36 for U36 nucleotide substitution. Furthermore, the hypermodified nucleoside at position 37 has been changed from t6A to ms2i6A.     

The linear electric field effect in stellacyanin, azurin and in some simple model compounds.

All mononuclear Cu(II) sites in frozen solution are non-centrosymmetric and, unless physically constrained, will have a tetrahedral distortion away from the usual square planar structure often presented for Cu(II) complexes. Blue copper sites such as are found in azurin and stellacyanin have a greater distortion towards a tetrahedral geometry than do simple Cu(II) complexes. The distortion is comparable to that which is observed for Cu(II)-o-phenanthroline dichloride, a known tetrahedral complex. Blue copper sites possess an axis of asymmetry directed away from g parallel which could arise from a metal-sulfur interaction.     

Linear electric field effect and electron spin-echo studies of uteroferrin. Evidence for iron coordination by a nitrogen-containing ligand

Uteroferrin and semimethemerythrin, proteins possessing spin-coupled binuclear iron centers, exhibit large linear electric field effects in their mixed-valence, EPR-active states. This indicates that the paramagnetic center of each protein is noncentrosymmetric and suggests that charge may be localized on one of the iron atoms. The magnetic field dependence of the linear electric field effects for both proteins demonstrates that the direction of most facile polarization of the binuclear iron centers is near the orientation giving rise to gmin. Electron spin-echo studies of uteroferrin reveal that its magnetic electron interacts with at least one and possibly two classes of nitrogen nuclei. Furthermore, comparison of echo envelope spectra for uteroferrin with that of ferric bleomycin suggests that one of these nuclei is from a histidine ligand.     

Effects of γ-acetylenic GABA and γ-vinyl GABA on electrically-induced spinal cord convulsions and on spinal cord GABA concentration

The effects of γ-acetylenic GABA and γ-vinyl GABA on electrically-induced spinal cord convulsions were compared to the effects of these same drugs on spinal cord GABA concentration. The data show that the effects of these two compounds on seizure activity do not correlate either positively or negatively with changes in GABA concentration. Although both drugs produced marked increases in the amount of GABA in the spinal cord, their effects on spinal cord convulsions were qualitatively different and failed to correlate temporally with alterations in GABA concentration.     

Multiple sites of vanadate and peroxovanadate action in Xenopus oocytes

In Xenopus laevis oocytes, the insulin mimics, vanadate and peroxovandates (PV), stimulated the uptake of ³H-2-deoxyglucose and incorporation of ³⁵S-methionine into protein. For both hexose transport and protein synthesis, peroxovandates (produced by reacting vandate and H₂O₂) were at least as potent as vandate. Microinjection of peroxovandates into the oocytes stimulated 2-deoxyglucose uptake. However, methionine incorporation was not stimulated by microinjection of peroxovanadate or vanadate solutions. Consistent with these results and with the possibility that vandate and peroxovandates enter the cell on a phosphate transporter, raising the medium phosphate concentration from 1 mM to 10 mM blocked vanadate-stimulated hexose transport and partially reduced peroxovanadates stimulation of hexose transport. Increased medium phosphate did not reduce stimulation of protein synthesis by either effector. Taken together, these data indicate that vanadate/peroxovanadates act at both intracellular and extracellular sites. Action at the former stimulates hexose uptake and action at the latter, protein synthesis. © 1995 Wiley-Liss, Inc.

1 This artilce is a US Government work and, as such, is in the public domain in the United States of America.

     

Characterization of missense suppressors of a double mutant of the tryptophan synthetase alpha chain of Escherichia coli.

Summary Eight suppressors of trpA218, a missense double mutant of trpA, the gene for the tryptophan synthetase alpha chain of Escherichia coli, have been further characterized genetically, physiologically and biochemically. trpA218 possesses an inactive alpha chain that contains leucine (instead of phenylalanine) at position 22 and serine (instead of glycine) at position 211. Replacement of either mutant amino acid by the corresponding wild type amino acid leads to an active alpha chain. To determine whether each trpA218 suppressor (Su218) affects the 22 or 211 position, a substitute trpA218 was constructed. Whereas the original double mutant possesses a Ser211 specified by the codon AGU, we constructed a trpA(Leu22-Ser211) in which the Ser211 codon is UCG. All eight Su218s failed to suppress the new double mutant. The suppressors fall into two classes according to growth in various media. Six of the eight map in the region of glyV, a gene for the GGU/C-reading glycine tRNA. After reversed phase column chromatography of radioactively labeled glycyl-tRNA, the suppressor tRNAs exhibited altered profiles that were similarly different from the parental tRNA in all eight cases. These results suggest that there are several classes of Su218, that all of them suppress the serine codon AGU (or AGC) corresponding to position 211, and that at least six of the eight are mutationally altered glycine tRNAs.A preliminary report of portions of this work was presented at the spring meeting of the Texas Branch of the American Society for Microbiology, College Station, Texas, March, 1975     

Ultrastructure of the Host-Pathogen Relationship in Entomosporium Leaf Spot Disease of Photinia

Entomosporium mespili appears to be a hemibiotroph on infected Photinia leaves. This fungal pathogen produced distinctive haustoria in living host cells in young lesions. Each haustorium possessed a long slender neck with a single septum and an enlarged distal body that contained a single nucleus. A collar of host cell wall material was associated with the haustorial neck. Intact haustoria also were found in necrotic cells of older lesions. However, by this stage of disease development, the pathogen also possessed an extensive system of branched, septate hyphae that grew indiscriminately between and through dead and dying host cells. These hyphae eventually gave rise to a subcutaneous layer of sporogenous cells that formed conidia.