var1 Gene on the mitochondrial genome of Torulopsis glabrata

We have cloned and sequenced a region of the Torulopsis glabrata mitochondrial genome homologous to the Saccharomyces cerevisiae var1 gene (var1Sc). An open reading frame that could encode a protein of 339 amino acids was found with 72.7% amino acid and 85.3% nucleotide sequence homology to the S. cerevisiae var1 gene. The T. glabrata gene (var1Tg) is transcribed yielding two stable RNAs, a more abundant 13.5 S RNA and a less abundant 18 S species. We have also identified a candidate for a T. glabrata var1 protein among mitochondrial translation products labeled in isolated mitochondria. The var1Tg gene is even more A + T-rich (93%) than var1Sc (89.6%) and has conserved the strong codon bias of var1Sc. Major differences between the two sequences were found. Significant among these are that no GC clusters are found in var1Tg and the sequences surrounding each of the sites where known polymorphisms exist in var1Sc have deletions at the corresponding sites in var1Tg. These data are discussed with respect to possible origins of these var1 genes and translocation of GC clusters in S. cerevisiae mitochondrial DNA.     

Promoter mutations affecting divergent transcription in the Tn10 tetracycline resistance determinant

The tetracycline resistance determinant in transposon Tn10 consists of two genes, the tetA resistance gene and the tetR repressor gene, that are transcribed from divergent overlapping promoters. We determined the levels of pulse-labeled tet messenger RNA in Escherichia coli strains with the Tn10 tet genes on a multicopy plasmid. Addition of the inducer 5a,6-anhydrotetracycline results in a 270- to 430-fold increase in tetA mRNA and a 35- to 65-fold increase in tetR mRNA. As judged by the relative molar amounts of tetA and tetR mRNA synthesized under maximally inducing conditions, the tetA promoter (tetPA) is 7 to 11 times more active than the two tetR promoters (tetPR1 and tetPR2) combined. We characterized ten mutations in tetPA, including nine single-base-pair substitutions and a 30-base-pair deletion. All of the single-base-pair changes reduce the agreement with the consensus sequence for promoters recognized by E. coli RNA polymerase. Mutations in highly conserved nucleotides result in a 200- to 600-fold reduction in tetPA activity in vivo. Unexpectedly, tetPA mutations reduce by two- to fourfold the combined activity in vivo of tetPR1 and tetPR2, in spite of their locations outside the -35 and -10 regions of tetPR1 and tetPR2. For two tetPA mutations, the negative effect on tetPR activity was also demonstrated in tetR- tetPR-lacZ operon fusion strains, thus eliminating the possibility that it is due to variations in either plasmid copy-number or induction efficiency. The pleiotropic effects of tetPA mutations are discussed in terms of the expectation that the overlapping tet promoters compete for RNA polymerase.     

Genetic reconstruction and functional analysis of the repeating lipoyl domains in the pyruvate dehydrogenase multienzyme complex of Escherichia coli

The dihydrolipoamide acetyltransferase component (E2p) of the pyruvate dehydrogenase complex of Escherichia coli contains three highly homologous sequences of about 100 residues that are tandemly repeated to form the N-terminal half of the polypeptide chain. All three sequences include a lysine residue that is a site for lipoylation and they appear to form independently folded functional domains. These lipoyl domains are in turn linked to a much larger (about 300 residues) subunit-binding domain of the E2p chain that aggregates to form the octahedral inner core of the complex and also contains the acetyltransferase active site. In order to investigate whether individual lipoyl domains play different parts in the enzymic mechanism, selective deletions were made in vitro in the dihydrolipoamide acetyltransferase gene (aceF) so as to excise one or two of the repeating sequences. This was facilitated by the high degree of homology in these sequences, which allowed the creation of hybrid lipoyl domains that closely resemble the originals. Pyruvate dehydrogenase complexes incorporating these genetically reconstructed E2p components were purified and their structures were confirmed. It was found that the overall catalytic activity, the system of active site coupling, and the ability to complement pyruvate dehydrogenase complex mutants, were not significantly affected by the loss of one or even two lipoyl domains per E2p chain. No special role can be attached thus far to individual lipoyl domains. On the other hand, certain genetic deletions affecting the acetyltransferase domain caused inactivation of the complex, highlighting particularly sensitive areas of that part of the E2p chain.     

ATP-induced ΔpH formation in chloroplast ATP synthase proteoliposomes

Summary A procedure to reconstitute CF₀CF₁ proteoliposomes by gel filtration through a Sephadex-column pre-equilibrated with valinomycin and potassium is described. Proteoliposomes reconstituted by this procedure catalyze an ATP-induced pH of 2.5 to 3.5 units. pH was measured with either 9-aminoacridine or with the pH indicator pyranine trapped inside the proteoliposomes. CF₀CF₁ proteoliposomes prepared by conventional techniques catalyzed an ATP-induced formation, but were unable to catalyze an ATP-induced pH even in the presence of valinomycin.The ATP-induced pH was sensitive to uncouplers and energy transfer inhibitors and was increased at low temperatures. It is suggested that ATP-induced pH was observed in these proteoliposomes due to the efficient removal of intravesicular ammonium introduced with the CF₀CF₁ preparation. The ammonium acted as an internal buffer, and thus prevented an observable pH formation.     

Lens cell-to-cel channel protein: II. Conformational change in the presence of calmodulin

Summary Lens fibers are coupled by communicating junctions, clusters of cell-to-cell channels composed of a 28-kD intrinsic membrane protein (MIP26). Evidence suggests that these and other cell-to-cell channels may close as a result of protein conformational change induced by activated calmodulin. To test the validity of this hypothesis, we have measured the intrinsic fluorescence emission and far-ultraviolet circular dichroism of the isolated components MIP26, calmodulin, and the MIP26-calmodulin complex, both in the absence and presence of Ca⁺⁺, an uncoupling agent. MIP26 shows no change in either, fluorescence emission (primarily tryptophan and a measure of aromatic constitutivity) or in its circular dichroism spectrum. Calmodulin exhibits a 32% increase in fluorescence emission intensity with constant emission wavelength, entirely tyrosine, and a 44% increase in -helicity, changes previously described. The MIP26-calmodulin complex, on the other hand, displays fluorescence emission and circular dichroism spectra which are slightly different from the sum of the two single components, but shows marked differences in both spectra upon Ca⁺⁺ addition. This indicates a change in conformation in one or both of the two components. Spectral changes include a 5-nm blue-shift, a 50% increase in tyrosine fluorescene emission, a 25% decrease in tryptophan fluorescence emission, and a 5% increase in the -helicity of the complex. These changes also occur about an isosbestic point and are fully reversible. These data provide additional evidence that activated calmodulin may modulate gating of cell-to-cell channels by affecting channel protein.     

The mechanistic nature of the membrane potential dependence of sodium-sugar cotransport in small intestine

Summary Methods are described which demonstrate the use of unidirectional influx of¹⁴C-tetraphenylphosphonium (¹⁴C-TPP⁺) into isolated intestinal epithelial cells as a quantitative sensor of the magnitude of membrane potentials created by experimentally imposed ion gradients. Using this technique the quantitative relationship between membrane potential () and Na⁺-dependent sugar influx was determined for these cells at various Na⁺ and -methylglucoside (-MG) concentrations. The results show a high degree of dependence for the transport Michaelis constant but a maximum velocity for transport which is independent of . No transinhibition by intracellular sugar (40mm) can be detected. Sugar influx in the absence of Na⁺ is insensitive to 1.3mm phlorizin and independent of . The mechanistic implications of these results were evaluated using the quality of fit between calculated and experimentally observed kinetic constants for rate equations derived from several transport models. The analysis shows that for models in which translocation is the potential-dependent step the free carrier cannot be neutral. If it is anionic, the transporter must be functionally asymmetric. A model in which Na⁺ binding is the potential-dependent step (Na⁺ well concept) also provides an appropriate kinetic fit to the experimental data, and must be considered as a possible mechanistic basis for function of the system.     

Ecological aspects of a silicified bivalve fauna from the Silurian of Gotland

The silicified Wenlockian (Silurian) bivalve fauna from MÖllbos, Gotland, is part of a life assemblage. The vast number of shells show unusual phenomena, e.g. shell repair, pearl and tumour formation, etc. A number of shells contain epibionts and bored, round holes. Presumptive predators of the bivalve community are discussed. Size-frequency distribution of the two most abundant species possibly reflects age classes. The fauna, comprising eleven species, is dominated by deposit-feeders (90 %). They exhibit niche diversification, including at least three different feeding levels within the sediment.     

The frequency of mutants in human fibroblasts UV-irradiated at various times during S-phase suggests that genes for thioguanine- and diphtheria toxin-resistance are replicated early

Human cells deficient in rate of excision repair of DNA damage induced by UV-radiation, i.e., xeroderma pigmentosum (XP) cells, are much more sensitive to the mutagenic effect of UV than are cells from normal persons. The lower frequency of mutants in the latter cells has been attributed to the fact that, unlike XP cells, they excise most of the potentially mutagenic lesions before these can be converted into mutations. If semi-conservative DNA synthesis on a template still containing unexcised lesions is responsible for introducing mutations and if replication of the gene of interest, e.g., hypoxanthine (guanine)phosphoribosyltransferase (HPRT) for thioguanine resistance or the elongation factor 2 (EF-2) for diphtheria toxin resistance, occurs at a particular time during S-phase, it should be possible to shorten the time available for such repair by synchronizing cells and irradiating them just as the gene is to be replicated. The predicted result would be a much higher frequency of mutants at one part in the S-phase than at other times. To test this, cells were synchronized using the alpha-polymerase inhibitor aphidicolin, which blocks cells at the G1/S border. Autoradiography, cytofluorimetry, and incorporation of tritiated thymidine studies showed that DNA synthesis started immediately after release from aphidicolin and was completed in 8-10 h. Cells irradiated with 6 J/m2 at various times post-release were assayed for survival and mutations. The frequency of thioguanine- or diphtheria toxin-resistant cells in the population was highest in cells irradiated during the first fifth of the S-phase, i.e., 0-1.5 h post-release. It was significantly lower in cells irradiated at later times. In contrast, UV-induced cytotoxicity showed no significant time dependence during S-phase. These data suggest that the HPRT and EF-2 genes are replicated early in S-phase.