Plasmid rolling circle replication and its control

Abstract This review summarises current information on rolling circle replicating plasmids originally isolated from Gram-positive bacteria with a low guanine and cytosine content in their DNA. It focuses on the peculiar biological features of these small, high copy number plasmids that replicate via an asymmetric RC mechanism. The regulation of plasmid copy number is also discussed.     

The aspartate aminotransferase gene family of Arabidopsis encodes isoenzymes localized to three distinct subcellular compartments

Here, a complete study is described of all the genes and isoenzymes for aspartate aminotransferase (AspAT) present in Arabidopsis thaliana . Four classes of cDNAs representing four distinct AspAT genes ( ASP1—ASP4 ) have been cloned from Arabidopsis . Sequence analysis of the cDNAs suggests that the encoded proteins are targeted to different subcellular compartments. ASP1 encodes a mitochondrial form of AspAT, ASP3 encodes a chloroplastic/plastidic form of AspAT, whereas ASP2 and ASP4 each encode cytosolic forms of AspAT. Three distinct AspAT holoenzymes (AAT1—AAT3) were resolved by activity gel analysis. Organelle isolation reveals that AAT1 is mitochondrial-localized, AAT3 is plastid-localized, and AAT2 is cytosolic. Gene-specific Northern analysis reveals that each Asp mRNA accumulates differentially with respect to organ-type. However, the individual Asp mRNAs show no dramatic fluctuations in response to environmental stimuli such as light. Southern analysis reveals that four distinct nuclear genes probably represent the entire AspAT gene family in Arabidopsis . These molecular studies shed light on the subcellular synthesis of aspartate in Arabidopsis and suggest that some of the AspAT isoenzymes may play overlapping roles in plant nitrogen metabolism.     

The karyotype and ultrastructural characteristics of spontaneous preimplantation mouse parthenotes

Karyotypic and light and electron microscopical analyses were made of spontaneous preimplantation mouse parthenotes from the LT/Sv inbred strain. It was found that the activated oocyte and developing embryos were diploid. We believe that diploidization is achieved by the oogonium undergoing a premeiotic mitosis without cytokinesis followed by two meiotic divisions, thus producing diploid parthenotes. The developmental events with respect to membrane specialization, such as junctional complexes, were similar to those observed in fertilized embryos. A unique feature of the developing parthenote was the failure of the mitochondria to change during the morula stage. The mitochondria retained a few irregularly oriented cristae rather than many transversely oriented ones observed in morulae developing from fertilized eggs. The significance of this observation is discussed.     

Variation of mitochondrial volume fraction along multiply innervated fibers in rabbit extraocular muscle

Mitochondrial volume fraction was compared among three regions along the length of six multiply innervated fibers (MIFs) in the orbital surface layer of rabbit superior rectus. These MIFs are of about 5 μm diameter toward the middle of their length, and of about 15 μm diameter toward their proximal and distal ends. The region of highest volume fraction (26%) was located toward the proximal end of their segment of minimal diameter, in apparent association with endplate-like nerve junctions. The region of lowest volume fraction (8%) was located at their distal segment of maximal diameter. The region toward the distal end of their segment of minimal diameter displayed an intermediate volume fraction (15%). These mitochondrial volume fractions were further analyzed in terms of the relative contributions of the I-band, the A-band, and the subsarcolemmal mitochondrial clusters. Comparable changes in mitochondrial content occur in both the I-band and A-band: in the fibers' distal segment of maximal diameter, however, the mitochondrial volume fraction in the A-band (5%) is lower than in the I-band (11%). These modifications of mitochondrial content along the fibers' length occur irrespective of the contributions of the subsarcolemmal mitochondrial clusters.     

Rhesus monkey micro mixed lymphocyte and mitogen reactivity: Optimal conditions and normal variability

Optimal conditions for the rhesus monkey micro mixed lymphocyte system with multiple automated harvesting of samples were evaluated. Parameters studied were cell concentration, length of culture period, methods of inactivation of cell populations, supplementation of media, type of culture plates, and changes in the reactivity of cells from individual animals over an extended time period. This work was supported in part by Portland Veterans Administration Hospital, Portland, Oregon, and the General Research Support Branch of the U.S. Public Health Service Grant RR00163, the Bureau of Medicine and Surgery, Navy Department, Work Unit No. M4318. 01.007ABG2. The opinions or assertions contained herein are the private ones of the authors and are not to be construed as official or reflecting the views of the U.S. Navy Department or the Naval service at large. The animals used in this study were handled in accordance with the provisions of Public Law 89–54 as amended by Public Law 91–579, “Animal Welfare Act of 1970,” and the principles outlined in the “Guide for the Care of Laboratory Animals,” U.S. Department of Health, Education, and Welfare Publication No. (NIH) 73-23.     

Dence for a gene in rats affecting lymphocyte responsiveness to PHA

A gene controlling high responsiveness of lymphocytes to in vitro stimulation by PHA was transferred from the Lewis strain of rats to the BN background by ten generations of backcrossing. The high-responder phenotype was initially defined on the basis of incorporation of³H-thymidine, but we show that this trait also involves higher levels of mitotic activity than are observed with low responder lymphocytes. This gene is not closely linked to any histocompatibility locus which could be detected by skin grafting, and it does not appear to affect the proportion of T lymphocytes.     

H-Y antigen in X,i(Xq) gonadal dysgenesis: Evidence of X-linked genes in testicular differentiation

Summary Three years ago, we detected H-Y antigen in the white blood cells of a phenotypic female with several of the stigmata of Turner's syndrome, and the mosaic karyotype: 45,X/46,X,i(Xq). We surmised at the time that the isochromosome, i(Xq), may have contained occult Y-chromosome-derived material. We have now confirmed the presence of H-Y in this patient and we have obtained evidence for the presence of H-Y in four of five other similar patients, all of whom are notable for carrying at least a single cell line with the karyotype 46,X,i(Xq). Although we cannot categorically exclude the presence of Y-chromosomal genes in the cells of these patients, there is no cytogenetic evidence of structural rearrangement involving the Y in any of the cases. Expression of H-Y antigen in association with i(Xq) thus implies that H-Y structural genes are X-situated, or alternatively that they are autosomal and X-regulated. It would follow that the H-Y⁺ cellular phenotype per se is not a valid marker for the Y-chromosome, and that H-Y genes that have been mapped to the pericentric region of the Y may be regulatory.     

Assembly of the Mitochondrial Membrane System: Nuclear Suppression of a Cytochrome b Mutation in Yeast Mitochondrial DNA

In a previous study, a mitochondrial mutant expressing a specific enzymatic deficiency in co-enzyme QH₂-cytochrome c reductase was described (Tzagoloff, Foury and Akai 1976). Analysis of the mitochondrially translated proteins revealed the absence in the mutant of the mitochondrial product corresponding to cytochrome b and the presence of a new low molecular weight product. The premature chain-termination mutant was used to obtain suppressor mutants with wild-type properties. One such revertant strain was analyzed genetically and biochemically. The revertant was determined to have a second mutation in a nuclear gene that is capable of partially suppressing the original mitochondrial cytochrome b mutation. Genetic data indicate that the nuclear mutation is recessive and is probably in a gene coding for a protein involved in the mitochondrial translation machinery.