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This Week in PNAS: May 15, 2013

In the early, online edition of the Proceedings of the National Academy of Sciences, an international team led by investigators in New Zealand presents findings from metagenomics-based functional analyses of soil and rock samples from an ice-free area in the Antarctic. The researchers used GeoChip — an array targeting variants in genes from hundreds of functional categories — in their assessment of samples from McKelvey Valley, one of the Antarctic's so-called McMurdo Dry Valleys. Results of these experiments highlight key functional pathways in the samples, which varied depending on the substrate considered. The search also uncovered pathways related to temperature, osmotic, and nutrient related stressors, study authors say, "offering tangible clues to the mechanisms behind the enduring success of microorganisms in this seemingly inhospitable terrain."

Major depressive disorder corresponds with changes in circadian rhythm-related gene expression in the human brain, according to work by researchers at the University of Michigan, the University of California, and elsewhere. The team performed array-based gene expression analyses on post-mortem brain samples from nearly three dozen individuals with MDD and 55 unaffected controls, incorporating information related to each individual's time of death. From this data, investigators were able to track down hundreds of transcripts with expression profiles that vary over the span of a day in various brain regions. These expression cycles appeared to askance in samples from individuals with MDD, they found, with expression peaks that were less pronounced and had somewhat different timing.

Exposure to warm temperatures can allow the expression of certain genes that would otherwise remain silent in Arabidopsis, researchers from China, the US, and France found. Using deep sequencing and other experiments, the team determined that Arabidopsis plants grown at warmer temperatures tend to have lower levels of post-transcriptional gene silencing — an apparent consequence of epigenetic changes that alter expression of the suppressor of gene silencing 3, or SGS3, protein and lead to a dip in the production of small, interfering RNAs. The epigenetic factors affecting this process are apparently inherited across Arabidopsis generations, researchers note, pointing to a "previously undescribed association between warming temperatures, an epigenetic system, and siRNA biogenesis."