H2AX post-translational modifications in the ionizing radiation response and homologous recombination

Histone H2AX phosphorylation on a C-terminal serine residue to form “γ-H2AX” is a critical early event in the chromatin response to chromosomal DNA double strand breaks in eukaryotes. In mammalian cells, γ-H2AX is formed when H2AX is phosphorylated on serine 139 by ATM or by other DNA damage response kinases. H2AX prevents genomic instability and tumorigenesis, and supports class-switch recombination at immunoglobulin heavy chain loci in mammals. We showed previously that H2AX controls double strand break repair by homologous recombination (HR) between sister chromatids. the HR functions of H2AX are mediated by interaction of γ-H2AX with the chromatin-associated adaptor protein MDC1. H2AX is potentially subject to additional post-translational modifications associated with the DNA damage response and with other chromatin functions. To test this idea, we used mass spectroscopy to identify H2AX residues additional to serine 139 that are post-translationally modified following exposure of cells to ionizing radiation (IR) and identified several new IR-responsive residues of H2AX. We determined the impact of IR-responsive H2AX residues on cellular resistance to IR and on H2AX-dependent HR, and also analyzed the contribution to HR of other known or potential post-translationally modified residues of H2AX. The results suggest that the HR and IR-resistance functions of H2AX are controlled in large part by specific MDC1-interacting residues of H2AX, but that additional H2AX residues modulate these core functions of H2AX.Key words: H2AX, homologous recombination, ionizing radiation, double strand break repair, histone, histone code, post-translational modification, chromatin, DNA repair     

Photosynthetic oxygen production and community respiration in the Indian sector of the Antarctic Ocean during austral summer

Photosynthetic oxygen production by phytoplankton and community respiration in the Indian sector of the Antarctic Ocean were estimated from changes in oxygen concentrations in light and dark bottles. Gross production varied between 0.1 and 5.1 µmol O₂ l^-1 day^-1. In the same water, community respiration (the sum of oxygen consumption by heterotrophs and phytoplankton) was 0.4-3.6 µmol O₂ l^-1 day^-1, which accounted for 47-343% of the gross production. Algal and heterotrophic respirations were distinguished using some assumptions. These estimates showed that heterotrophic respiration accounted for most of the community respiration (70-91% depending upon the assumptions), indicating that heterotrophic respiration plays an important role in the mineralization of phytoplankton production in the surveyed sea area. Gross production rate correlated with chlorophyll a concentration, showing that the photosynthetic production rate of oxygen depends on the abundance of phytoplankton. Moreover, there was a significant relationship between gross production and community respiration rates. These regression equations suggested that negative net production occurred under the usually low concentration of chlorophyll observed in the Indian sector of the Antarctic Ocean. Hence, the net exchange of carbon dioxide due to biological processes through the sea surface seemed to be not as large as expected in the Antarctic Ocean, although the number of data were limited at this stage.     

DIMETHYLSULFONIOPROPIONATE PRODUCTION BY PSYCHROPHILIC DIATOM ISOLATES1

Dimethylsulfoniopropionate (DMSP) production by psychrophilic diatom strains, Chaetoceros sp., Navicula sp., and Nitzschia sp., were experimentally analyzed. All strains showed rapid growth (0.3–0.5 d^?1) under cold culture conditions (1.7°C). DMSP concentrations (both as particulate, DMSP_p, and dissolved, DMSP_d) were negligible in cultures of Chaetoceros sp., whereas those for Navicula sp. and Nitzschia sp. increased toward the end of the 56‐day experiments. The ratio of DMSP_p:chl a of the latter two species was approximately 5 in the early exponential growth phase and decreased slightly toward the early stationary phase. During the stationary phase, when chl a and fluorescence remained constant, the ratios in both species increased linearly by up to approximately 6.5 times the value at the start of the stationary phase. This growth‐related DMSP production by diatoms may result in the low concentrations of DMSP_p observed in the early stage of diatom blooms under natural conditions.     

Hemolytic Activity of a Togavirus,Getah*

A purified toga-alphavirus, Getah (GET), showed optimal hemolytic activity for one-day-old chick red blood cells when incubated at 37 C for 120 min at pH 6.2. Experimental data obtained from various angles, such as pH dependency, inhibition by virus-specific antiserum and by concanavalin A, indicated that the hemolysis was a property of the virus particle itself. Although the mechanism of hemolysis by togaviruses has not been known, our results indicated that viral lipids may participate in this activity since the hemolytic activity was impaired by delipidation procedures.