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This Week in PNAS: Mar 27, 2018

Researchers from the University of North Carolina describe a DNA repair map developed for the yeast model organism Saccharomyces cerevisiae following exposure to ultraviolet light, which produces two characteristic DNA lesions repaired by nucleotide excision. Using a modified version of the excision-repair sequencing, or XR-seq, the team looked at repair of cyclobutane pyrimidine dimer and pyrimidine-pyrimidone photoproducts across the yeast genome, within individual genes, and in transcribed sequences. "Our data reveal a dual-incision mode in yeast that differs from that in humans and show the sequential repair of the transcribed strand and non-transcribed strand," the authors say, pointing to ties between excision repair levels and genes' transcription rates. 

A team from Germany, the Netherlands, and Belgium present an in vitro method for doing genetic mapping in hybrid mammals without breeding or transgenesis. The approach relied on the production of hybrid embryonic stem cells, followed by drug-induced mitotic cross-over — a strategy the researchers applied to embryonic stem cells produced from Mus musculus and M. spretus mouse species, which belong to lineages that split an estimated 1.5 million years ago. After spurring on a flurry of mitotic cross-overs with a drug called ML216, they mapped resistance to the anti-metabolite drug tioguanine in the M. musculus-M. spretus hybrid embryonic stem cells.

Finally, investigators from Canada and Germany explore stress signaling in half a dozen lineages of streptophyte algae, which are part of a green algae class that's thought to be most closely related to the ancestral algae that spawned land plants. The team did comparative RNA sequencing on six streptophyte algae, followed by de novo transcriptome assembly, tracing the activity of some 46,000 genes as the algal species were exposed to cold stress, unusually high light levels, and other growth conditions. "Our results show that streptophyte algae respond to cold and high light stress via expression of hallmark genes used by land plants," the authors report, noting that plastid genes appear particularly prone to differential regulation under stressful conditions in the streptophyte algae.