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Quorum sensing is a term used to describe cell-to-cell communication that allows cell-density-dependent gene expression. Many bacteria use acyl-homoserine lactone (acyl-HSL) synthases to generate fatty acyl-HSL quorum-sensing signals, which function with signal receptors to control expression of specific genes. The fatty acyl group is derived from fatty acid biosynthesis and provides signal specificity, but the variety of signals is limited. Here we show that the photosynthetic bacterium Rhodopseudomonas palustris uses an acyl-HSL synthase to produce p-coumaroyl-HSL by using environmental p-coumaric acid rather than fatty acids from cellular pools. The bacterium has a signal receptor with homology to fatty acyl-HSL receptors that responds to p-coumaroyl-HSL to regulate global gene expression. We also found that p-coumaroyl-HSL is made by other bacteria including Bradyrhizobium sp. and Silicibacter pomeroyi. This discovery extends the range of possibilities for acyl-HSL quorum sensing and raises fundamental questions about quorum sensing within the context of environmental signalling.

Methylobacterium populi sp. nov., a novel aerobic, pink-pigmented, facultatively methylotrophic, methane-utilizing bacterium isolated from poplar trees (Populus deltoides6nigra DN34) Species of the genus Methylobacterium are strictly aerobic, facultatively methylotrophic, Gram-negative, rod-shaped bacteria that are able to grow on one-carbon compounds (e.g. methanol or methylamine), as well as on a variety of C 2 , C 3 and C 4 substrates (Green, 1992). Only the type species, Methylobacterium organophilum, has been shown to use methane as the sole source of carbon and energy (Patt et al., 1976). The genus Methylobacterium belongs to the a2 subclass of the Proteobacteria and currently consists of 14 species with validly published names (Heumann, 1962;Ito & Iizuka, 1971;Kouno & Ozaki, 1975;Patt et al., 1976;Rock et al., 1976;Austin & Goodfellow, 1979;Urakami & Komagata, 1984;Bousfield & Green, 1985;Green et al., 1988;Urakami et al., 1993; Wood et al., 1998; Doronina et al., 2000 Doronina et al., , 2002 McDonald et al., 2001;Sy et al., 2001). Members of the genus Methylobacterium are distributed in a wide variety of natural and man-made environments, including soil, air, dust, freshand marine water and sediments, water supplies, bathrooms, air-conditioning systems and masonry (Hiraishi et al., 1995;Trotsenko et al., 2001). Some species have been described as opportunistic human pathogens (Truant et al., 1998; Hornei et al., 1999). In addition, methylotrophic bacteria are frequently associated with terrestrial and aquatic plants, where they colonize roots and leaf surfaces (Austin et al., 1978;Yoshimura, 1982;Corpe & Rheem, 1989;Trotsenko et al., 2001;Lidstrom & Chistoserdova, 2002). The association of Methylobacterium species with plants seems to rely on a symbiotic relationship between the bacterium and the plant host. Plants produce methanol The GenBank/EMBL/DDBJ accession number for the 16S and 16S-23S IGS rDNA sequence of strain BJ001 T is AY251818.Tissue culture images, photomicrographs, 16S and 16S-23S IGS rDNA sequences of BJ001 T , sequence similarity matrices, carbon-and nitrogen-source utilization data and enzymic reactions of BJ001 T are available as supplementary material in IJSEM Online.Abbreviations: IGS, intergenic spacer; SEM, scanning electron microscope.

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The rate-limiting enzyme of anaerobic benzoate degradation by Rhodopseudomonas palustris, benzoyl coenzyme A (CoA) reductase, is highly sensitive to oxygen, and its synthesis is tightly regulated. We determined that a previously unknown gene in the benzoate gene cluster, badM, encodes a transcriptional repressor of benzoyl-CoA reductase gene expression. BadM controls gene expression from the benzoyl-CoA reductase promoter in concert with two previously described transcriptional activators.

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Although isolated on 4-aminobenzoate, Burkholderia cepacia strain PB4 is also able to grow on 4-nitrobenzoate. Degradation of an equimolar mixture of the nitroaromatic compound 4-nitrobenzoate and its corresponding aminoaromatic derivative 4-aminobenzoate by this strain was investigated. Batch experiments showed that, irrespective of preculturing conditions, both compounds were degraded simultaneously. The mixture-degrading ability of B. cepacia strain PB4 was subsequently tested in continuous packed bed reactors (PBR) with the strain immobilized on Celite grade R-633 or R-635. Higher degradation rates were achieved with the larger particles of Celite R-635. Maximum simultaneous degradation rates per liter of packed bed of 0.925 mmol 1(-1) h(-1) 4-nitrobenzoate and 4-aminobenzoate were obtained for an applied loading rate of the same value (0.925 mmol 1(-1) h(-1) of each compound). Even when the applied load was not removed in its entirety, neither of the two compounds was degraded preferentially but a percentage of both of them was mineralized. The present study shows the possibility for a pure strain to biodegrade not only a nitroaromatic compound (4-nitrobenzoate) but also its corresponding amino derivative (4-aminobenzoate) continuously and simultaneously.