Highlights d 35 fungal species were isolated from the ocean, sponge, coral, and coastal sediment d Time-lapse imaging reveals unusual cell cycles, cell-division patterns, and polarity

Fungi have been found in every marine habitat that has been explored, however, the diversity and functions of fungi in the ocean are poorly understood. In this study, fungi were cultured from the marine environment in the vicinity of Woods Hole, MA, USA including from plankton, sponge and coral. Our sampling resulted in 36 unique species across 20 genera. We observed many isolates by time-lapse differential interference contrast (DIC) microscopy and analyzed modes of growth and division. Several black yeasts displayed highly unconventional cell division cycles compared to those of traditional model yeast systems. Black yeasts have been found in habitats inhospitable to other life and are known for halotolerance, virulence, and stress-resistance. We find that this group of yeasts also shows remarkable plasticity in terms of cell size control, modes of cell division, and cell polarity. Unexpected behaviors include division through a combination of fission and budding, production of multiple simultaneous buds, and cell division by sequential orthogonal septations. These marine-derived yeasts reveal alternative mechanisms for cell division cycles that seem likely to expand the repertoire of rules established from classic model system yeasts.

In eukaryotes, gene expression depends on chromatin organization. However, how chromatin affects the transcription dynamics of individual RNA polymerases has remained elusive. Here, we use dual trap optical tweezers to study single yeast RNA polymerase II (Pol II) molecules transcribing along a DNA template with two nucleosomes. The slowdown and the changes in pausing behavior within the nucleosomal region allow us to determine a drift coefficient, χ , which characterizes the ability of the enzyme to recover from a nucleosomal backtrack. Notably, χ can be used to predict the probability to pass the first nucleosome. Importantly, the presence of a second nucleosome changes χ in a manner that depends on the spacing between the two nucleosomes, as well as on their rotational arrangement on the helical DNA molecule. Our results indicate that the ability of Pol II to pass the first nucleosome is increased when the next nucleosome is turned away from the first one to face the opposite side of the DNA template. These findings help to rationalize how chromatin arrangement affects Pol II transcription dynamics.T o accommodate the massive amount of genetic material within the nucleus, DNA is packaged into chromatin. The level of chromatin compaction determines the accessibility of the underlying DNA, which in turn impacts gene expression (1). The first step in gene expression is transcription, where RNA polymerase II (Pol II) processively moves along the DNA template to generate an RNA copy. However, how chromatin impacts the translocation dynamics of individual RNA polymerases has remained unclear.The fundamental unit of chromatin is a single nucleosome, which consists of 147 bp of DNA wrapped ∼ 1.7 times around a histone octamer (2). In vitro experiments have revealed that a single nucleosome exhibits a significant mechanical barrier to the transcribing Pol II (3-7). Using optical tweezers, it was shown that a nucleosome both decreases the rate of forward translocation and increases polymerase pausing and backtracking (3, 5). These changes have been suggested to depend on the unwrapping dynamics of the nucleosome itself, which are governed by nucleosomal properties such as the histones' modification state as well as the underlying DNA sequence (3, 5).In vivo, the process of chromatin transcription is far more complex. Chromatin accessibility is affected by many accessory factors that facilitate the progression of RNA polymerases through nucleosomal obstacles (1). A central outstanding question is to what extent the nucleosomal arrangement affects Pol II transcription behavior. This arrangement might be particularly important in budding yeast, an organism with a compact genome with short internucleosomal distances (8-11).Here, we study the impact of the arrangement of two nucleosomes on the nucleosomal transcription performance of individual molecules of Pol II. We use dual-trap optical tweezers in a single-molecule approach to follow single enzymes of Pol II as they progress through a dinucleosomal array (3, 5). Specifi...