Obligately microaerophilic, nitrogen-fixing bacteria were found associated with roots of Spartina afternflora Loisel and in root-associated sediments from salt marshes in Nova Scotia, Canada, and Georgia. These bacteria differ from previously described species and thus represent a new species. The cells of all strains which we studied are small, rigid, curved, motile, and rod shaped and have single polar flagella. Metabolism is respiratory, and the strains utilize organic and amino acids, but not carbohydrates, as sole carbon sources. Poly-p-hydroxybutyrate is not produced. These traits and the guanine-plus-cytosine contents of the deoxyribonucleic acids of these strains (28.3 ? 0.1 mol%) indicate that they are members of the genus Campyfobacter Sebald and Veron 1963. However, these strains can be distinguished from the previously described species of Campylobacter by the presence of nitrogenase, by their tolerance of and apparent requirement for NaC1, by the production of pigment from tryptophan, by a combination of other biochemical traits, and by their association with plant roots. Therefore, we propose that these strains represent a new species, Campylobacter nitrojigifis, and we designate strain CI (= ATCC 33309) as the type strain.Campyfobacter species are commonly occurring pathogens and commensal organisms that are associated with a diverse range of animal hosts (23, 24,31). Of particular interest in this genus is the Campyfobacter jejuni-Campylobacter cofi group (Campyfobacter fetus subsp. jejuni), which increasingly is being recognized as a cause of enteritis in humans (3, 10). In addition to the human-and animal-associated campylobacters, free-living strains of Campyfobacter have been isolated in culture (11, 15), but the taxonomic status of these organisms has not been established yet. One of the free-living strains, strain CIT (type strain), was found associated with the roots of Spartina afternflora Loisel growing in a salt marsh near Halifax, Nova Scotia, Canada; this strain was found to be capable of fixing nitrogen (15). In this paper we describe the isolation in culture of several additional nitrogen-fixing Campyfobacter strains from S. afternflora roots and root-associated sediments from a salt marsh on Sapelo Island, Georgia. We propose that these nitrogen-fixing, Spartina-associated strains represent a new species, Campyfobacter nitrojigifis. MATERIALS AND METHODSIsolation. Roots of S. altevnifora Loisel were taken from a salt marsh on Sapelo Island, Georgia, and were washed free of sediment in adjacent creek water. Root pieces (length, 2 cm) were placed in tubes of semisolid diazotroph medium (15). Other tubes were seeded with root-associated sediment samples. After 72 h at room temperature, the cultures were transferred to fresh tubes of semisolid malate-salts (MS) medium (15), and nitrogenase activity was determined by acetylene reduction assays (4). After 24 h, tubes of MS medium were capped with serum stoppers, and acetylene was injected to give a final partial pressure of C2Hz of 2 kPa...

In Arabidopsis thaliana, catalase is encoded by a small gene family. We have characterized cDNA and genomic clones containing the Arabidopsis catalase gene CAT3, present as a single copy in the nuclear genome. Six introns were identified in the CAT3 coding region and two transcription start sites have been been mapped by primer extension. The deduced amino acid sequence of CAT3 is highly similar to other catalases. CAT3 expression is similar in seedlings germinated and grown either in continuous light or in continuous dark, suggesting that CAT3 expression in seedlings is not light responsive. CAT3 expression is controlled by the circadian clock; in 5-week-old plants grown on a light-dark cycle and then transferred to continuous light, robust oscillations in CAT3 mRNA abundance with circadian period persist for at least five circadian cycles. Interestingly, the peak in CAT3 mRNA abundance occurs in the subjective evening, which is out of phase with expression of the Arabidopsis CAT2 catalase gene, which shows clock-regulated expression gated to the subjective early morning.

Hai H o n g Zhong,all A n d r e w S. Resnick,a,2 M a r t i n Straume,b a n d C. R o b e r t s o n M~C l u n g~.~ a Department of Biological Sciences, Dartmouth College, 6044 Gilman Laboratory, Hanover, New Hampshire 03755 and Metabolism, University of Virginia, Charlottesville, Virginia 22903 National Science Foundation Center for Biological Timing, Department of Interna1 Medicine, Division of Endocrinology Persistent oscillation in constant conditions is a defining characteristic of circadian rhythms. However, in plants transferred into extended dark conditions, circadian rhythms in mRNA abundance commonly damp in amplitude over two or three cycles to a steady state level of relatively constant, low mRNA abundance. In Arabidopsis, catalase CAT3 mRNA oscillations damp rapidly in extended dark conditions, but unlike catalase CAT2 and the chlorophyll a/b binding protein gene CAB, in which the circadian oscillations damp to low steady state mRNA abundance, CAT3 mRNA oscillations damp to high steady state levels of mRNA abundance. Mutational disruption of either phytochrome-or cryptochromemediated light perception prevents damping of the oscillations in CAT3 mRNA abundance and reveals strong circadian oscillations that persist for multiple cycles in extended dark conditions. Damping of CAT3 mRNA oscillations specifically requires phytochrome A but not phytochrome B and also requires the cryptochromel blue light receptor. Therefore, we conclude that synergistic signaling mediated through both phytochrome A and cryptochromel is required for damping of circadian CAT3 mRNA oscillations in extended dark conditions.

Entrainment, the synchronization of a circadian clock with the external environment, is a crucial step in daily life. Although many signals contribute to entrainment, light and temperature are typically the strongest resetting cues. Much progress has been made concerning light resetting in the model plant Arabidopsis thaliana . Multiple photoreceptors (phytochromes, cryptochromes, LOV-domain proteins) are involved in light perception. The clock genes CCA1 , LHY and TOC1 are all probable targets of light signalling, although the details of these pathways are not completely established. Temperature can entrain the clock, but little is known about the mechanism underlying this resetting; no obvious clock gene candidate for temperature resetting has been identified. Although circadian research has emphasized oscillations in free-running conditions, in the real world the circadian clock is entrained. During entrainment, short or long period mutants exhibit a 24-h period, but a mutant phenotype is often manifested as an altered phase relationship with the entraining cycle; short and long period mutants show leading and lagging phases, respectively, and this may be detrimental under some conditions. Arrhythmic CCA1-overexpressing plants display increased lethality under very short photoperiods, consistent with the circadian clock being of adaptive significance to life on a rotating world.