SecA protein: Autoregulated initiator of secretory precursor protein translocation across theE. coli plasma membrane

Several classes ofsecA mutants have been isolated which reveal the essential role of this gene product forE. coli cell envelope protein secretion. SecA-dependent,in vitro protein translocation systems have been utilized to show that SecA is an essential, plasma membrane-associated, protein translocation factor, and that SecA's ATPase activity appears to play an essential but as yet undefined role in this process. Cell fractionation studies suggested that SecA protein is in a dynamic state within the cell, occurring in soluble, peripheral, and integral membraneous states. These data have been used to argue that SecA is likely to promote the initial insertion of secretory precursor proteins into the plasma membrane in a manner dependent on ATP hydrolysis. The protein secretion capability of the cell has been shown to translationally regulatesecA expression with SecA protein serving as an autogenous repressor, although the exact mechanism and purpose of this regulation need to be defined further.     

Ammonium effects on streptonigrin biosynthesis byStreptomyces flocculus

Summary A defined medium containing glucose and ammonium as the sole carbon and nitrogen sources was developed to support growth and streptonigrin production. In this defined medium, increased initial levels of ammonium resulted in increased growth suggesting that nitrogen is the growth limiting nutrient. In some cases, increased initial ammonium levels resulted in decreased specific streptonigrin productivity, suggesting that nitrogen regulatory mechanisms may adversely affect streptonigrin biosynthesis. This suggestion that nitrogen regulation adversely affects antibiotic biosynthesis is further supported by results from two studies in which the ammonium supply to the cells was controlled. In the first study, streptonigrin productivity and final titer were enhanced by the addition of an ammonium trapping agent. In the second experiment, when ammonium chloride was fed slowly throughout the course of cultivation, the production phase was lengthened and the maximum antibiotic concentration was enhanced compared to the batch controls containing either the same initial or the same total ammonium chloride levels. Although our results indicate streptonigrin production may be subject to nitrogen regulatory mechanisms, the effect of nitrogen on streptonigrin production cannot be strictly correlated to the extracellular ammonium concentration. In fact, we observed that when ammonium was depleted from the medium, streptonigrin production ceased.     

Fine root demography in alfalfa (Medicago sativa L.)

In perennial forages like alfalfa (Medicago sativa L.), repeated herbage removal may alter root production and mortality which, in turn, could affect deposition of fixed N in soil. Our objective was to determine the extent and patterns of fine-diameter root production and loss during the year of alfalfa stand establishment. The experiment was conducted on a loamy sand soil (Udorthentic Haploboroll) in Minnesota, USA, using horizontally installed minirhizotrons placed directly under the seeded rows at 10, 20, and 40 cm depths in four replicate blocks. We seeded four alfalfa germplasms that differed in N₂ fixation capacity and root system architecture: Agate alfalfa, a winter hardy commercially-available cultivar; Ineffective Agate, which is a non-N₂-fixing near isoline of Agate; a new germplasm that has few fibrous roots and strong tap-rooted traits; and a new germplasm that has many fibrous roots and a strongly branched root system architecture. Video images collected biweekly throughout the initial growing season were processed using C-MAP-ROOTS software.More than one-half of all fine roots in the upper 20 cm were produced during the first 7 weeks of growth. Root production was similar among germplasms, except that the highly fibrous, branch-rooted germplasm produced 29% more fine roots at 20 cm than other germplasms. In all germplasms, about 7% of the fine roots at each depth developed into secondarily thickened roots. By the end of the first growing season, greatest fine root mortality had occurred in the uppermost depth (48%), and least occurred at 40 cm (36%). Survival of contemporaneous root cohorts was not related to soil depth in a simple fashion, although all survivorship curves could be described using only five rates of exponential decline. There was a significant reduction in fine root mortality before the first herbage harvest, followed by a pronounced loss (average 22%) of fine roots at the 10- and 20-cm depths in the 2-week period following herbage removal. Median life spans of these early-season cohorts ranged from 58 to 131 days, based on fitted exponential equations. At all depths, fine roots produced in the 4 weeks before harvest (early- to mid-August) tended to have shorter median life spans than early-season cohorts. Similar patterns of fine root mortality did not occur at the second harvest. Germplasms differed in the pattern, but not the ultimate extent, of fine root mortality. Fine root turnover during the first year of alfalfa establishment in this experiment released an estimated 830 kg C ha^–1 and 60 kg N ha^–1, with no differences due to N₂ fixation capacity or root system architecture.     

Higher level relationships of Apiales (Apiaceae and Araliaceae) based on phylogenetic analysis of rbcL sequences

The two families of the order Apiales (Apiaceae and Araliaceae) represent a classic example of the difficulty in understanding evolutionary relationships between tropical-temperate family pairs. In Apiales, this problem is further compounded by phylogenetic confusion at almost every taxonomic level, including ordinal, interfamilial, and infrafamilial, due largely to difficulties in understanding trends in morphological evolution. Phylogenetic analyses of rbcL sequences were employed to resolve relationships at the ordinal and familial levels. The results of the ordinal analysis confirm the placement of Apiales in an expanded subclass Asteridae as the sister group to Pittosporaceae, and refute the traditional alliance of Apiales with Cornales and Rosidae. This study has also resolved relationships of a number of enigmatic genera, suggesting, for example, that Melanophylla, Aralidium, Griselinia, and Toricellia are close relatives of Apiales. Clarification of phylogenetic relationships has concomitantly provided insights into trends of morphological evolution, and suggests that the ancestral apialean taxon was probably bicarpellate, simple-leaved, woody, and paleotropical. Phylogenetic analysis at the family level suggests that apiaceous subfamily Hydrocotyloideae, often envisioned as an intermediate group between Apiaceae and Araliaceae, is polyphyletic, with some hydrocotyloids closely allied with Araliaceae rather than Apiaceae. With the exception of some hydrocotyloids, Apiaceae appear to be monophyletic. The relationship between Apiaceae and Araliaceae remains problematic. Although the shortest rbcL trees suggest that Apiaceae are derived from within a paraphyletic Araliaceae, this result is only weakly supported.     

Plasmid transfer to indigenous marine bacterial populations by natural transformation

Abstract Horizontal gene transfer among microbial populations has been assumed to occur in the environment, yet direct observations of this phenomenon are rare or limited to observations where the mechanism(s) could not be explicitly determined. Here we demonstrate the transfer of exogenous plasmid DNA to members of indigenous marine bacterial populations by natural transformation, the first report of this process for any natural microbial community. Ten percent of marine bacterial isolates examined were transformed by plasmid DNA while 14% were transformed by chromosomal DNA. Transformation of mixed marine microbial assemblages was observed in 5 of 14 experiments. In every case, acquisition of the plasmid by members of the indigenous flora was accompanied by modification (probably from genetic rearrangement or methylation) that altered its restriction enzyme digestion pattern. Estimation of transformation rates in estuarine environments based upon the distribution of competency and transformation frequencies in isolates and mixed populations ranged from 5 × 10⁻⁴ to 1.5 transformants/1 day. Extrapolation of these rates to ecosystem scales suggests that natural transformation may be an important mechanism for plasmid transfer among marine bacterial communities.     

Mutations in the peptidyl transferase region of E. coli 23S rRNA affecting translational accuracy.

We have produced mutations in a cloned Escherichia coli 23S rRNA gene at positions G2252 and G2253. These sites are protected in chemical footprinting studies by the 3' terminal CCA of P site-bound tRNA. Three possible base changes were introduced at each position and the mutations produced a range of effects on growth rate and translational accuracy. Growth of cells bearing mutations at 2252 was severely compromised while the only mutation at 2253 causing a marked reduction in growth rate was a G to C transversion. Most of the mutations affected translational accuracy, causing increased readthrough of UGA, UAG and UAA nonsense mutations as well as +1 and -1 frameshifting in a lacZ reporter gene in vivo. C2253 was shown to act as a suppressor of a UGA nonsense mutation at codon 243 of the trpA gene. The C2253 mutation was also found not to interact with alleles of rpsL coding for restrictive forms of ribosomal protein S12. These results provide further evidence that nucleotides localized to the P site in the 50S ribosomal subunit influence the accuracy of decoding in the ribosomal A site.     

Anaerobic Degradation of Normal- and Branched-Chain Fatty Acids with Four or More Carbons to Methane by a Syntrophic Methanogenic Triculture

Syntrophic degradation of normal- and branched-chain fatty acids with 4 to 9 carbons was investigated with a mesophilic syntrophic isobutyrate-butyrate-degrading triculture consisting of the non-spore-forming, syntrophic, fatty acid-degrading, gram-positive rod-shaped strain IB, Methanobacterium formicicum T1N, and Methanosarcina mazei T18. This triculture converted butyrate and isobutyrate to methane and converted valerate and 2-methylbutyrate to propionate and methane. This triculture also degraded caproate, 4-methylvalerate, heptanoate, 2-methylhexanoate, caprylate, and pelargoate. During the syntrophic conversion of isobutyrate and butyrate, a reversible isomerization between butyrate and isobutyrate occurred; isobutyrate and butyrate were isomerized to the other isomeric form to reach nearly equal concentrations and then their concentrations decreased at the same rates. Butyrate was an intermediate of syntrophic isobutyrate degradation. When butyrate was degraded in the presence of propionate, 2-methylbutyrate was synthesized from propionate and isobutyrate formed from butyrate. During the syntrophic degradation of valerate, isobutyrate, butyrate, and 2-methylbutyrate were formed and then degraded. During syntrophic degradation of 2-methylbutyrate, isobutyrate and butyrate were formed and then degraded.     

Biological destruction of CCl(4): I. Experimental design and data

A denitrifying consortium capable of transforming carbon tetrachloride (CCl(4)) was cultured from aquifer sediment from the U.S. Department of Energy's Hanford Site in southeastern Washington State. To understand the kinetics of the biological destruction of CCl(4) by these microbes, a set of experiments, the conditions of which were chosen according to a fractional factorial experimental design, were completed. This article reports on the experimental design along with the results for CCl(4), biomass, acetate, nitrate, and nitrite concentrations. These data indicate that growth is inhibited by high nitrite concentrations, whereas CCl(4) degradation is slowed by the presence of nitrate and/or nitrite. (c) 1994 John Wiley & Sons, Inc.