Salmonella typhimurium encounters a variety of acid conditions during both its natural and pathogenic existence. The ability of this organism t o respond transcriptionally to low pH is an area of active interest but little knowledge. As part of an ongoing investigation of low-pH adaptation, 18 pH-controlled IacZ operon fusions in Salmonella typhimurium have been identified (15 in this study) and categorized into a t least 11 different loci. They include iroA (at 57 min), aciA (99 min), aciB (90-93 min), aciD (ompC, 45 min), acil, aciK (33-36 rnin), aniC (93 min), anil(33-36 min), hyd (59 min), cadA (54 min) and aniG (63 rnin). All but two were induced by low pH. One of the exceptions, the iron-regulated iroA locus, was induced a t high pH. The unusual aciA locus was induced by low pH under semiaerobic conditions but high pH under aerobic conditions. Most of the other aci genes were expressed best under anaerobic conditions. Many of these genes exhibited strict co-inducer requirements for small molecules to be expressed in minimal medium. These included iron for iroA, tyrosine for aniC, I and aciK, mannose for aniG, formate for hyd, lysine for cadA, and unknown components of complex medium for aciA, aciB and aciD. Six regulatory circuits were revealed involving a t least five regulatory loci (fur, oxrG, earAB, earC and ompR). As part of the adaptive response to low pH, 5. typhimurium will induce an acid protection system called the acid tolerance response (ATR). As has been shown for fur mutations, the oxrG regulatory mutation interfered with the normal induction of this system.

SummaryThe pOp / LhG4 transcription factor system was used to determine whether the synthetic pOp promoter, integrated at one position in the Arabidopsis genome, could be efficiently and faithfully activated by the heterologous transcription factor, LhG4, expressed in a variety of different patterns. This is a precondition for the development and exploitation of large collections of LhG4 activation lines that direct predictable tissue-specific expression of transgenes. We selected a pOp-GUS reporter insertion that was efficiently activated after crossing to an activator line that expressed the synthetic transcription factor LhG4 from the Cauliflower Mosaic Virus 35S promoter. This reporter line, pOp-GUS(g2), was then combined with activator loci that expressed LhG4 from one of seven different promoters, each with a different tissue specificity. pOp-GUS(g2) was activated faithfully in combination with six of these seven activator constructs, but generated an unexpected expression pattern in combination with the seventh construct, a fusion to a cyclin promoter (CYC-LhG4). The aberrant expression pattern could be attributed to the pOp-GUS(g2) insertion site, as the CYC-LhG4 activator lines directed the expected pattern of expression from a second pOp-GUS insertion. These results show that it is feasible to construct an activator collection in which LhG4 is expressed from diverse promoters or enhancer traps, but that individual pOp reporter loci can vary in their competence to respond to certain activator patterns. We discuss the implications for the design and use of mis-expression technology in Arabidopsis .

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Artemisia tridentata ssp. tridentata is the dominant and defining shrub in the Great Basin Desert, with well-documented allelopathic tendencies that have generally been ascribed to its most abundantly released secondary metabolites. However, as a minor component, sagebrush releases a highly biologically active substance, methyljasmonate (MeJA), which is known to function as both a germination inhibitor and promoter in laboratory studies. Nicotiana attenuata is a tobacco species native to the Great Basin Desert and grows in newly burned juniper-sagebrush habitats for 2-3 yr following a fire. With a combination of field and laboratory studies, we examined the role of MeJA release from sagebrush by both air and water transport in inhibiting N. attenuata seed germination. We demonstrated that sagebrush interacts allelopathically with the seed bank of N. attenuata through its release of MeJA. In the field, seeds buried 0-40 cm from sagebrush plants for 4 months in net bags had significantly reduced germination compared to seeds buried similarly but protected in plastic bags. Moreover, germination on soils collected from underneath sagebrush plants was reduced by 60% compared to seeds placed on soils collected between sagebrush plants or outside of the sagebrush population. Exposure to A. tridentata seeds and seedlings did not affect N. attenuata germination, suggesting that established sagebrush plants only influence the tobacco's seed bank. In the laboratory, exposure of seeds to sagebrush emissions resulted in germination delays of up to 6 d. Exposure to volatile and aqueous MeJA also inhibited germination of N. attenuata seeds at quantities that are released naturally by sagebrush: 3.5 microg/hr and 1.12 microg/seed cup (56 ng/seed), respectively. A. tridentata seeds were significantly more resistant to MeJA, being inhibited at 336 microg MeJA (16.8 microg/seed), 300 times greater than the level of aqueous MeJA required to inhibit N. attenuata seeds. MeJA inhibited N. attenuata germination regardless of the seed's dormancy state and the specific epimer (trans- or cis-) of MeJA. Germination on sagebrush chaff that had been heated to reduce MeJA content was negatively correlated with the amount of MeJA remaining in the chaff. Germination of a nondormant, conspecific tobacco, N. trigonophylla, which grows in the same area but is not associated with fire, is less sensitive than N. attenuata to the extracts of sagebrush litter, but similarly sensitive to MeJA. Additionally, four of five other tobacco species that are not known to be associated with sagebrush are less sensitive to MeJA, suggesting an evolved sensitivity to MeJA. To determine the proportion of germination inhibition of a sagebrush extract that could be attributed to MeJA, we serially diluted sagebrush extracts with water and restored the quantity of MeJA of the original extract by adding appropriate quantities of synthetic MeJA; 16-60% of the inhibitory activity of the original extract could be attributed to the MeJA. We conclude that MeJA release from sa...