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The kinetics of gravitropism and of amyloplast sedimentation were studied in dark-grown protonemata of the moss Ceratodon purpureus (Hedw.) Brid. The protonemata grew straight up at a rate of 20–25 m·h– in nutrient-supplemented agar. After they were oriented to the horizontal, upward curvature was first detected after 1–1.5 h and reached 84° by 24 h. The tip cells exhibited an amyloplast zonation, with a tip cluster of nonsedimenting amyloplasts, an amyloplast-free zone, and a zone with pronounced amyloplast sedimentation. This latter zone appears specialized more for lateral than for axial sedimentation since amyloplasts sediment to the lower wall in horizontal protonemata but do not fall to the basal wall in vertical protonemata. Amyloplast sedimentation started within 15 min of gravistimulation; this is within the 12–17-min presentation time. The data support the hypothesis that some amyloplasts function as statoliths in these cells.This work was supported by the National Aeronautics and Space Administration grant NAGW-780. We thank Professor E. Hartmann and J. Schwuchow for providing Ceratodon cultures, Dr. John Z. Kiss and Jeff Young for valuable discussions, and Professor Rainer Hertel (University of Freiburg, FRG) for bringing this material to our attention. 相似文献
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Fred J. Genthner Janeshwar Upadhyay Robert P. Campbell Barbara R. Sharak Genthner 《Microbial ecology》1990,20(1):283-288
Culturable counts of antibiotic resistant, genetically engineeredPseudomonas fluorescens were determined on antibiotic-containing plate count agar during starvation in water. Prior to starvation, colony counts
obtained on all media separated into two groups. The mean of the colony counts on plate count agar with or without tetracycline
(4.9 × 106 ml−1) was significantly higher than the mean colony counts on plate count agar containing either nalidixic acid or nalidixic acid
plus tetraclycline (2.5×106 ml−1). After 20 days of starvation the highest mean colony counts continued to be obtained on plate count agar (7.2 × 106 ml−1) with slightly, but significantly, lower counts obtained on plate count agar containing either nalidixic acid (5.6 × 106 ml−1) or tetraclycline (1.5×106 ml−1). A combination of nalidixic acid and tetracycline in plate count agar, however, dramatically reduced colony counts (8.3
× 102 ml−1) after this starvation period. The addition of catalase to plate count agar containing nalidixic acid and tetracycline negated
the effect caused by this combination of antibiotics. When colony counts obtained over the entire 20 day incubation were considered,
the addition of MgSO4 to plate count agar containing nalidixic acid and tetracycline resulted in a significant increase in colony counts. Other
combinations of antibiotics, nalidixic acid+carbenicillin, nalidixic acid+kanamycin, streptomycin+tetracycline, streptomycin+carbenicillin,
rifampicin+tetracycline, rifampicin+carbenicillin, and rifampicin+kanamycin, did not inhibit colony formation of starved cells.
Antibiotic resistant strains ofP. putida andEscherichia coli also displayed sensitivity to the combination of nalidixic acid and tetracycline in plate count agar after starvation. 相似文献
59.
The chloroplast enzyme phosphoribulokinase is reversibly deactivated by oxidation of Cys16 and Cys55 to a disulfide. Although not required for catalysis, Cys16 is an active-site residue positioned at the nucleotide-binding domain (Porter and Hartman, 1988). The hyperreactivity of Cys16 has heretofore limited further active-site characterization by chemical modification. To overcome this limitation, the partially active enzyme,S-methylated at Cys16, has been probed with a potential affinity reagent. Treatment of methylated enzyme with bromoacetylethanolamine phosphate results in essentially complete loss of catalytic activity. Inactivation follows pseudo-first-order kinetics and exhibits a rate saturation with an apparentK d of 3–4 mM. ATP, but not ribulose 5-phosphate, affords substantial protection. Complete inactivation correlates with incorporation of 1 mol of [14C]reagent per mole of enzyme subunit. Amino acid analysis of the [14C]-labeled enzyme demonstrates that only cysteine is modified, and mapping of tryptic digests shows that Cys55 is a major site of alkylation. These results indicate that Cys55 is also located in the ATP-binding domain of the active-site. 相似文献
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Earl F. Albone Fred K. Hagen Claude Szpirer Lawrence A. Tabak 《Glycoconjugate journal》1996,13(5):709-716
Mucin glycoproteins are a major constituent of salivary secretions and play a primary role in the protection of the oral cavity. Rat submandibular glands (RSMG) synthesize and secrete a low molecular weight (114 kDa) mucin glycoprotein. We have isolated, partially sequenced, and characterized the gene which encodes the RSMG apomucin. The gene is encoded by three exons of 106 nt, 69 nt, and 991 nt, separated by introns of 921 nt and 12.5 kb. CAAT and TATA elements are present, at –68 and –26, respectively, in the 5 flanking sequence of the RSMG apomucin gene. The tandem repeat domain present in exon III consists of ten tandem repeats of 39 nt encoding the consensus sequence PTTDSTTPAPTTK. Sequence comparison and organization of the nucleic acid sequence encoding the tandem repeats of two alleles for this gene suggests that the apomucin gene has undergone recombinational events during its evolution. No significant sequence similarity was found with other mucin genes, or with other known salivary gland-specific genes. The gene was localized to rat chromosome 14 using somatic cell hybrids that segregate rat chromosomes. Since this, to our knowledge, represents the first RSMG mucin gene cloned, we have designated this geneMucsmg.Abbreviations RSMG
rat submandibular gland
- RSM
rat salivary mucin
- GRP
glutamine-glutamic-acid rich protein
- nt
nucleotide
- kb
kilobase
Sequences reported herein have been assigned GenBank accession numbers U33441 and U33442. 相似文献