Elucidation of Gephyronic Acid Biosynthetic Pathway Revealed Unexpected SAM-Dependent Methylations

Gephyronic acid, a cytostatic polyketide produced by the myxobacterium <i>Cystobacter violaceus</i> Cb vi76, exhibits potent and selective eukaryotic protein synthesis inhibition. Next-generation sequencing of the <i>C. violaceus</i> genome revealed five type I polyketide synthases and post-PKS tailoring enzymes including an <i>O</i>-methyltransferase and a cytochrome P450 monooxygenase. Seven methyltransferase (MT) domains embedded within the PKS subunits were found to install the methyl branches throughout the gephyronic acid skeleton. A rare loading domain from the GNAT superfamily also contains an embedded MT domain that catalyzes the in situ production of an isobutyryl starter unit. Phylogenetic analysis identified new motifs that distinguish MT domains located in PKS pathways with <i>in cis</i> acyltransferase (AT) domains from MT domains located in PKS pathways with <i>trans</i> AT enzymes. The identification of the gene cluster sets the stage for the generation of a heterologous expression system, which will allow further investigation of selective eukaryotic protein synthesis inhibitors through the generation of gephyronic acid analogues

Hollow Cathode Plasma-Enhanced Atomic Layer Deposition of Silicon Nitride Using Pentachlorodisilane

In this work, a novel chlorodisilane precursor, pentachlorodisilane (PCDS, HSi₂Cl₅), was investigated for the growth of silicon nitride (SiN_<i>x</i>) via hollow cathode plasma-enhanced atomic layer deposition (PEALD). A well-defined self-limiting growth behavior was successfully demonstrated over the growth temperature range of 270–360 °C. At identical process conditions, PCDS not only demonstrated approximately >20% higher growth per cycle than that of a commercially available chlorodisilane precursor, hexachlorodisilane (Si₂Cl₆), but also delivered a better or at least comparable film quality determined by characterizing the refractive index, wet etch rate, and density of the films. The composition of the SiN_<i>x</i> films grown at 360 °C using PCDS, as determined by X-ray photoelectron spectroscopy, showed low O content (∼2 at. %) and Cl content (<1 at. %; below the detection limit). Fourier transform infrared spectroscopy spectra suggested that N–H bonds were the dominant hydrogen-containing bonds in the SiN_<i>x</i> films without a significant amount of Si–H bonds originating from the precursor molecules. The possible surface reaction pathways of the PEALD SiN_<i>x</i> using PCDS on the surface terminated with amine groups (−NH₂ and −NH−) are proposed. The PEALD SiN_<i>x</i> films grown using PCDS also exhibited a leakage current density as low as 1–2 nA/cm² at 2 MV/cm and a breakdown electric field as high as ∼12 MV/cm