As part of a long-term campaign to document, study, and conserve the Heroes tapestries from The Cloisters collection at The Metropolitan Museum of Art, organic colorant analysis of Julius Caesar (accession number 47.101.3) was performed. Analysis with liquid chromatography–quadrupole time-of-flight mass spectrometry (LC-qToF-MS) revealed the presence of several multiply chlorinated xanthones produced only by certain species of lichen. Various lichen dye sources have been documented in the literature for centuries and are classified as either ammonia fermentation method (AFM) or boiling water method (BWM) dyes based on their method of production. However, none of these known sources produce the distinctive metabolites present in the tapestry. LC-qToF-MS was also used to compare the chemical composition of the dyes in the tapestry with that of several species of crustose lichen. Lichen metabolites, including thiophanic acid and arthothelin, were definitively identified in the tapestry based on comparison with lichen xanthone standards and a reference of Lecanora sulphurata, confirming the presence of a lichen source. This finding marks the first time that lichen xanthones have been identified in a historic object and the first evidence that BWM lichen dyes may have been used prior to the eighteenth century.

Many cancers use an alternative lengthening of telomeres (ALT) pathway for telomere maintenance. ALT telomeric DNA synthesis occurs in ALT-associated PML bodies (APBs). However, the mechanisms by which APBs form are not well understood. Here, we monitored the formation of APBs with time-lapse imaging employing CRISPR knock-in to track the promyelocytic leukemia (PML) protein at endogenous levels. We found APBs form via two pathways: telomeres recruit PML proteins to nucleate PML bodies de novo, or telomeres fuse with preformed PML bodies. Both nucleation and fusion of APBs require interactions between SUMO and SUMO interaction motifs (SIMs). Moreover, APB nucleation is associated with higher levels of SUMO and SUMO-mediated recruitment of DNA helicase BLM, resulting in more robust telomeric DNA synthesis. Finally, further boosting SUMO levels at telomeres enhances APB nucleation, BLM enrichment, and telomeric DNA synthesis. Thus, high SUMO levels at telomeres promote APB nucleation and stronger ALT activity.

The identification of natural yellow organic pigments known as yellow lakes presents an analytical challenge, due to their structural complexity, poor stability, the need for invasive sampling, and the capabilities of the existing analytical methods and instrumentation. In recent years, high-performance liquid chromatography, quadrupole time-of-flight mass spectrometry (HPLC-qToF-MS) has emerged as a powerful tool for museum scientists, allowing for high resolution, molecular level analysis of organic colorants in microscopic samples. To achieve deep, rich shadows in his paintings, Rembrandt mixed and layered translucent red and yellow lakes with opaque pigments; however, the composition of the yellow lakes had not been identified until the present study. To create dark translucent greens in the backgrounds of his compositions, Vermeer also chose yellow lake pigments mixed with blue and green. These yellow lakes have been difficult to identify until now. In this study, HPLC-qToF-MS was used to identify yellow lakes in two paintings by Vermeer and one by Rembrandt, to better understand the original appearance of the paint passages where these lakes were used, and to gain further insight into these artists’ techniques. In Aristotle with a Bust of Homer, several flavonoids and an anthraquinone from a Rhamnus species were identified in a painting by Rembrandt. To the best of our knowledge, this is the first time that Rhamnus has been unequivocally identified in a 17^th-century Dutch painting, despite being widely discussed in the historical literature. The analysis of samples from Vermeer’s Mistress and Maid and Study of a Young Woman detected a yellow lake based on weld, which was mixed with indigo and a copper-based green pigment. These pigment mixtures were originally dark translucent green, which have now discolored to brownish greens.