Genetic data suggest that the oligotrophic freshwater bacterium Caulobacter crescentus metabolizes D-xylose through a pathway yielding ␣-ketoglutarate, comparable to the recently described L-arabinose degradation pathway of Azospirillum brasilense. Enzymes of the C. crescentus pathway, including an NAD ؉ -dependent xylose dehydrogenase, are encoded in the xylose-inducible xylXABCD operon (CC0823-CC0819).

In the oligotrophic freshwater bacterium Caulobacter crescentus, D-xylose induces expression of over 50 genes, including the xyl operon, which encodes key enzymes for xylose metabolism. The promoter (P xylX ) controlling expression of the xyl operon is widely used as a tool for inducible heterologous gene expression in C. crescentus. We show here that P xylX and at least one other promoter in the xylose regulon (P xylE ) are controlled by the CC3065 (xylR) gene product, a LacI-type repressor. Electrophoretic gel mobility shift assays showed that operator binding by XylR is greatly reduced in the presence of D-xylose. The data support the hypothesis that there is a simple regulatory mechanism in which XylR obstructs xylose-inducible promoters in the absence of the sugar; the repressor is induced to release DNA upon binding D-xylose, thereby freeing the promoter for productive interaction with RNA polymerase. XylR also has an effect on glucose metabolism, as xylR mutants exhibit reduced expression of the Entner-Doudoroff operon and their ability to utilize glucose as a sole carbon and energy source is compromised.D-Xylose is the main constituent of xylan polymers, the major component of hemicellulose in plant cell walls. This pentose, in monomeric or polymerized form, is often present in habitats containing plant-derived biomass. The aquatic bacterium Caulobacter crescentus avidly consumes D-xylose as a carbon and energy source. D-Xylose metabolism in C. crescentus proceeds via a novel pathway initiated by the enzyme xylose dehydrogenase (XDH) (2, 24). The enzymes of this pathway are encoded in the xyl operon, whose expression is induced by D-xylose (9,16,24). In the work presented here, we identified the transcription factor controlling expression of the xyl operon and characterized the mechanism of regulation.For the past decade, the xyl operon promoter (P xylX ) has been the standard tool for engineering inducible gene expression in C. crescentus (1,10,12,15,20,25,26). This promoter is highly active when it is induced by addition of D-xylose to the medium and is virtually silent in the absence of xylose, provided that there is a single copy on the chromosome or it is in a low-copy-number plasmid vector. In its native chromosomal context, P xylX controls the expression of the five-gene xyl operon (CC0823 to CC0819) ( Fig. 1) (9). A putative operator sequence in P xylX was identified by virtue of its conservation in several xylose-regulated promoters (9). This operator overlaps the Ϫ10 region of P xylX (Fig. 1), suggesting that a DNA binding protein interacting with this site would interfere with RNA polymerase holoenzyme binding and/or transcription initiation. The simplest model for control of the xyl regulon is that a repressor ("XylR") binds to operator sites in the absence of xylose but dissociates from the operator when it is bound to xylose to allow gene expression. This model is consistent with the observation that a 4-bp mutation in the upstream half-site of the putative operator of P xylX , which did no...

Massive, long-lived deep-sea red tree corals (Primnoa pacifica) form a solid, layered axis comprised of calcite and gorgonin skeleton. They are abundant on the outer continental shelf and upper slope of the Northeast Pacific, providing habitat for fish and invertebrates. Yet, their large size and arborescent morphology makes them susceptible to disturbance from fishing activities. A better understanding of their growth patterns will facilitate in-situ estimates of population age structure and biomass. Here, we evaluated relationships between ages, growth rates, gross morphological characteristics, and banding patterns in 11 colonies collected from depths of ~141–335 m off the Alaskan coast. These corals ranged in age from 12 to 80 years old. They grew faster radially (0.33–0.74 mm year-1) and axially (2.41–6.39 cm year-1) than in previously measured older colonies, suggesting that growth in P. pacifica declines slowly with age, and that basal diameter and axial height eventually plateau. However, since coral morphology correlated with age in younger colonies (< century), we developed an in-situ age estimation technique for corals from the Northeast Pacific Ocean providing a non-invasive method for evaluating coral age without removing colonies from the population. Furthermore, we determined that annual bands provided the most accurate means for determining coral age in live-collected corals, relative to radiometric dating. Taken together, this work provides insight into P. pacifica growth patterns to inform coastal managers about the demographics of this ecologically important species. With this new ability to estimate the age of red tree corals in-situ, we can readily determine the age-class structure and consequently, the maturity status of thickets, using non-invasive video survey techniques when coupled with mensuration systems such as lasers or stereo-cameras. Enhanced surveys could identify which populations are most vulnerable to disturbance from human activities, and which should be highlighted for protection.

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