Streptomyces griseus, a well-known industrial producer of streptomycin, is a member of the genus Streptomyces, which shows a complex life cycle resembling that of fungi. A-factor, a C₁₃ γ-butyrolactone compound, was discovered as a self-regulatory factor or a bacterial hormone to induce morphological differentiation and production of secondary metabolites, including streptomycin, in this organism. Accumulating evidence has revealed an A-factor-triggered signal cascade, which is composed of several key steps or components. These include: (i) AfsA catalyzing a crucial step of A-factor biosynthesis, (ii) the A-factor-specific receptor (ArpA), which acts as a transcriptional repressor for adpA, (iii) adpA, a sole target of ArpA, which encodes a global transcriptional activator AdpA, and (iv) a variety of members of the AdpA regulon, a set of the genes regulated by AdpA. A-factor is biosynthesized via five reaction steps, in which AfsA catalyzes acyl transfer between a β-ketoacyl-acyl carrier protein and the hydroxyl group of dihydroxyacetone phosphate. The receptor ArpA, belonging to the TetR family, is a homodimer, each subunit of which contains a helix-turn-helix DNA-binding motif and an A-factor-binding pocket. The three-dimensional structure and conformational change upon binding A-factor are elucidated, on the basis of X-ray crystallography of CprB, an ArpA homologue. AdpA, belonging to the AraC/XylS transcriptional activator family, binds operators upstream from the promoters of a variety of the target genes and activates their transcription, thus forming the AdpA regulon. Members of the AdpA regulon includes the pathway-specific transcriptional activator gene strR that activates the whole streptomycin biosynthesis gene cluster, in addition to a number of genes that direct the multiple cellular functions required for cellular differentiation in a concerted manner. A variety of A-factor homologues as well as homologues of afsA/arpA are distributed widely among Streptomyces, indicating the significant role of this type of molecular signaling in the ecosystem and evolutional processes.

(Contributed by Teruhiko BEPPU, M.J.A.)

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Prostanoids, consisting of prostaglandins (PGs) and thromboxanes (TXs), are oxygenated products of C₂₀ unsaturated fatty acids. They include PGD₂, PGE₂, PGF_2α, PGI₂, and TXA₂. Given that aspirin-like nonsteroidal anti-inflammatory drugs exert their actions by suppressing prostanoid production, prostanoids have been implicated in processes inhibited by these drugs, including inflammation, fever, and pain. Prostanoids also contribute to vascular homeostasis, reproduction, and regulation of kidney and gastrointestinal functions. How prostanoids exert such a variety of actions had remained unclear, however. Prostanoids are released outside of cells immediately after their synthesis and exert their actions by binding to receptors on target cells. We have identified a family of eight types or subtypes of G protein–coupled receptors that mediate prostanoid actions. Another G protein–coupled receptor was also identified as an additional receptor for PGD₂. Genes for these receptors have been individually disrupted in mice, and analyses of these knockout mice have not only elucidated the molecular and cellular mechanisms of known prostanoid actions but also revealed previously unknown actions. In this article, I review the physiological and pathophysiological roles of prostanoids and their receptors revealed by these studies.

(Communicated by Tasuku HONJO, M.J.A.)

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We report the formation of water droplets by irradiating wet ambient air with deep UV light. The light sources were either a continuous low-pressure mercury lamp or pulsed ArF laser, which both emit light shorter than 200 nm. Water droplets were produced in reaction vessels under different temperature, relative humidity, and moisture-supply conditions. The particles grew as large as about 0.2 mm. The suggested mechanism is discussed with the photo-dissociations of oxygen and successively formed ozone, and further dark reactions giving hydrogen peroxide as a seeding nucleus. Observed concentrations of intermediates were well explained by simulating the proposed chemical reactions. A possible application to artificial rain is briefly described.

(Communicated by Kenichi HONDA, M.J.A.)

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