Postgrowth thermal annealing of an InGaN/GaN quantum-well sample with a medium level of nominal indium content ͑19%͒ was conducted. From the analyses of high-resolution transmission electron microscopy and energy filter transmission electron microscopy, it was found that thermal annealing at 900 °C led to a quasiregular quantum-dot-like structure. However, such a structure was destroyed when the annealing temperature was raised to 950 °C. Temperature-dependent photoluminescence ͑PL͒ measurements showed quite consistent results. Blueshift of the PL peak position and narrowing of the PL spectral width after thermal annealing were observed.
Among the indolocarbazole alkaloids of antitumor antibiotics, K-252a represents a structurally unique indolocarbazole glycoside and exhibits potent neuroprotective and broad anticancer activities. K-252a consists of K-252c and the unusual dihydrostreptose moiety, linked together with oxidative and glycosidic C-N bonds. Herein, we reported a complete sequence of an approximately 45 kb genomic fragment harboring the gene cluster for the biosynthesis of indolocarbazole alkaloids in Nocardiopsis sp. K-252 (NRRL15532). The sequence of 35 open reading frames discovered several new, critical genes, hence shedding new light on biosynthesis, resistance and regulation of K-252a and its analogs. To functionally characterize the gene cluster in vitro and in enzyme level, a multigene expression cassette containing the K-252c biosynthetic genes was constructed and successfully overexpressed in Escherichia coli to yield soluble proteins for cell-free tandem enzymatic assays. Consequently, the heterologous expression with soluble NokA and NokB led to in vitro production of chromopyrrolic acid (CPA), thereby providing functional evidence for K-252c biosynthesis. Moreover, a facile production of CPA in culture broth was successfully accomplished by using an in vivo biotransformation of L-tryptophan with E. coli harboring the gene cassette. Importantly, by sequence analysis and the functional characterization here and in the companion paper, biosynthetic pathways leading to formation of K-252a and its analogs were hence proposed. Together, the results provide critical information and materials useful for combinatorial biosynthesis of K-252a and its analogs for therapeutic applications.
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