The lys5 mutations of barley reveal the nature and importance of plastidial ADP-Glc transporters for starch synthesis in cereal endosperm

Much of the ADP-Glc required for starch synthesis in the plastids of cereal endosperm is synthesized in the cytosol and transported across the plastid envelope. To provide information on the nature and role of the plastidial ADP-Glc transporter in barley (Hordeum vulgare), we screened a collection of low-starch mutants for lines with abnormally high levels of ADP-Glc in the developing endosperm. Three independent mutants were discovered, all of which carried mutations at the lys5 locus. Plastids isolated from the lys5 mutants were able to synthesize starch at normal rates from Glc-1-P but not from ADP-Glc, suggesting a specific lesion in the transport of ADP-Glc across the plastid envelope. The major plastidial envelope protein was purified, and its sequence showed it to be homologous to the maize (Zea mays) ADP-Glc transporter BRITTLE1. The gene encoding this protein in barley, Hv.Nst1, was cloned, sequenced, and mapped. Like lys5, Hv.Nst1 lies on chromosome 6(6H), and all three of the lys5 alleles that were examined were shown to carry lesions in Hv.Nst1. Two of the identified mutations in Hv.Nst1 lead to amino acid substitutions in a domain that is conserved in all members of the family of carrier proteins to which Hv.NST1 belongs. This strongly suggests that Hv.Nst1 lies at the Lys5 locus and encodes a plastidial ADP-Glc transporter. The low-starch phenotype of the lys5 mutants shows that the ADP-Glc transporter is required for normal rates of starch synthesis. This work on Hv.NST1, together with the earlier work on BRITTLE1, suggests that homologous transporters are probably present in the endosperm of all cereals.     

Genotypic Differences of Candida albicans and C. dubliniensis Isolates Related to Ethnic/Racial Differences within the Same Geographic Area

Candida albicans and C. dubliniensis genotype differences among Israeli ethnic groups were assessed. Isolates from Jews (51), Arabs (35) and Druze (25) were genotyped. The distributions among ethnic groups were not different, however they differed (p = 0.002) from global populations. Therefore, C. albicans and C. dubliniensis genotype distribution differences in Israel are related to changes in all ethnic groups.     

Population Genetics of Streptococcus dysgalactiae Subspecies equisimilis Reveals Widely Dispersed Clones and Extensive Recombination

Background

Streptococcus dysgalactiae subspecies equisimilis (SDSE) is an emerging global pathogen that can colonize and infect humans. Although most SDSE isolates possess the Lancefield group G carbohydrate, a significant minority have the group C carbohydrate. Isolates are further sub-typed on the basis of differences within the emm gene. To gain a better understanding of their molecular epidemiology and evolutionary relationships, multilocus sequence typing (MLST) analysis was performed on SDSE isolates collected from Australia, Europe and North America.

Methodology/Principal Findings

The 178 SDSE isolates, representing 37 emm types, segregate into 80 distinct sequence types (STs) that form 17 clonal complexes (CCs). Eight STs recovered from all three continents account for >50% of the isolates. Thus, a small number of STs are highly prevalent and have a wide geographic distribution. Both ST and CC strongly correlate with group carbohydrate. In contrast, eleven STs were associated with >1 emm type, suggestive of recombinational replacements involving the emm gene; furthermore, 35% of the emm types are associated with genetically distant STs. Data also reveal a history of extensive inter- and intra-species recombination involving the housekeeping genes used for MLST. Sequence analysis of single locus variants identified through goeBURST indicates that genetic change mediated by recombination occurred ∼4.4 times more frequently than by point mutation.

Conclusions/Significance

A few genetic lineages with an intercontinental distribution dominate among SDSE causing infections in humans. The distinction between group C and G isolates reflects recent evolution, and no long-term genetic isolation between them was found. Lateral gene transfer and recombination involving housekeeping genes and the emm gene are important mechanisms driving genetic variability in the SDSE population.     

The initial step in human immunodeficiency virus type 1 GagProPol processing can be regulated by reversible oxidation

Background

Maturation of human immunodeficiency virus type 1 (HIV-1) occurs upon activation of HIV-1 protease embedded within GagProPol precursors and cleavage of Gag and GagProPol polyproteins. Although reversible oxidation can regulate mature protease activity as well as retrovirus maturation, it is possible that the effects of oxidation on viral maturation are mediated in whole, or part, through effects on the initial intramolecular cleavage event of GagProPol. In order assess the effect of reversible oxidation on this event, we developed a system to isolate the first step in protease activation involving GagProPol.

Methodology/Principal Findings

To determine if oxidation influences this step, we created a GagProPol plasmid construct (pGPfs-1C) that encoded mutations at all cleavage sites except p2/NC, the initial cleavage site in GagProPol. pGPfs-1C was used in an in vitro translation assay to observe the behavior of this initial step without interference from subsequent processing events. Diamide, a sulfhydral oxidizing agent, inhibited processing at p2/NC by >60% for pGPfs-1C and was readily reversed with the reductant, dithiothreitol. The ability to regulate processing by reversible oxidation was lost when the cysteines of the embedded protease were mutated to alanine. Unlike mature protease, which requires only oxidation of cys95 for inhibition, both cysteines of the embedded protease contributed to this inhibition.

Conclusions/Significance

We developed a system that can be used to study the first step in the cascade of HIV-1 GagProPol processing and show that reversible oxidation of cysteines of HIV-1 protease embedded in GagProPol can block this initial GagProPol autoprocessing. This type of regulation may be broadly applied to the majority of retroviruses.     

Ischemia dysregulates DNA methyltransferases and p16INK4a methylation in human colorectal cancer cells

Epigenetic modifications are involved in the initiation and progression of cancer. Expression patterns and activity of DNA methyltransferases (DNMTs) are strictly controlled in normal cells; however, regulation of these enzymes is lost in cancer cells due to unknown reasons. Cancer therapies which target DNMTs are promising treatments of hematologic cancers, but they lack effectiveness in solid tumors. Solid tumors exhibit areas of hypoxia and hypoglycaemia due to their irregular and dysfunctional vasculature, and we previously showed that hypoxia reduces global DNA methylation. Colorectal carcinoma (CRC) cells (HCT116 and 379.2; p53^+/+ and p53^-/-, respectively) were subjected to ischemia (hypoxia and hypoglycaemia) in vitro and levels of DNMTs were assessed. We found a significant decrease in mRNA for DNMT1, DNMT3a and DNMT3b, and similar reductions in DNMT1 and DNMT3a protein levels were detected by western blotting. In addition, total activity levels of DNMTs (as measured by an ELISA-based DNMT activity assay) were reduced in cells exposed to hypoxic and hypoglycaemic conditions. Immunofluorescence of HCT116 tumor xenografts demonstrated an inverse relationship between ischemia (as revealed by carbonic anhydrase IX staining) and DNMT1 protein. Bisulfite sequencing of the proximal promoter region of p16^INK4a showed a decrease in 5-methylcytosine following in vitro exposure to ischemia. These studies provide evidence for the downregulation of DNMTs and modulation of methylation patterns by hypoxia and hypoglycaemia in human CRC cells, both in vitro and in vivo. Our findings suggest that ischemia, either intrinsic or induced through the use of anti-angiogenic drugs, may influence epigenetic patterning and hence tumor progression.Key words: DNA methylation, DNA methyltransferases, colorectal carcinoma, ischemia, p53, hypoxia, hypoglycaemia