Book reviews

Books reviewed in this article:
A FUNCTIONAL BIOLOGY OF CROP PLANTS. By V. P. G utschick
MOLECULAR BIOLOGY OF CROP IMPROVEMENT. A Case Study of Wheat, Oilseed Rape, and Faba Beans. By R. B. A ustin , R. B. F lavell , I. E. H enson & H. J. B. L owe     

Creation of a test plasmid for detecting G-C-to-T-A transversions by changing serine to arginine in the active site of beta-lactamase.

Oligonucleotide-directed mutagenesis of the beta-lactamase gene, bla, on pBR322 was used to change the codon for the active-site serine 70, AGC, to CGC, coding for arginine. Escherichia coli cells carrying the mutant plasmid, pGD104, were sensitive to ampicillin, indicating that the arginine-containing enzyme is inactive. We characterized the reversion of the mutant bla gene by a number of mutagens and in different genetic backgrounds and demonstrated that full ampicillin resistance can be restored only by a G-C-to-T-A transversion occurring at the first base of the codon. Thus, reversion of the mutant bla gene is diagnostic for G-C-to-T-A transversions, and bacteria carrying pGD104 can be used as test strains to detect the occurrence of this mutation.     

Factors responsible for the differences in cultural estimates and direct microscopical counts of populations of bacterivorous nanoflagellates

Bacterivorous nanoflagellates (microflagellates) have been routinely enumerated in marine and freshwater samples using either a Most Probable Number (MPN) culture method or by a direct microscopical counting method (DC). These two techniques typically yield highly disparate estimates of the density of nanoflagellates in natural samples. We compared these methods with seawater and marine snow (macroscopic detrital aggregate) samples collected from surface waters throughout the North Atlantic and in freshwater samples collected at three stations in Lake Ontario. Densities of nanoflagellates determined by the two methods differed by as much as four orders of magnitude; the MPN estimate rarely exceeded 10% of the microscopical count, and averaged 1% of this count. The MPN estimate constituted a higher percentage of the DC value in environments with high concentrations of nanoflagellates relative to environments with low concentrations of nanoflagellates. The ratio of the culture count to the microscopical count (MPNDC) increased along an environmental gradient from oligotrophy to eutrophy, and was positively correlated with the density of bacteria in the samples. In laboratory experiments with two species of bacterivorous nanoflagellates, the MPN count constituted a much greater percentage of the DC count during the exponential growth phase of the nanoflagellate than during the stationary growth phase. Differences in the estimates of nanoflagellate density obtained with these two techniques probably can be explained by the trophic mode of these protozoa, their growth stage, and the amenability of these species to laboratory culture.     

Competitive yeast fermentation with selective flocculation and recycle

Continuous fermentations were carried out involving competition between two strains of Saccharomyces cerevisiae. One of the strains has a lower specific growth rate and is very flocculent, whereas the fastergrowing strain is nonflocculent. The product stream from the chemostat was fed into an inclined settler where the flocculent strain was partially separated from the nonflocculent strain as a result of the higher sedimentation rate of the flocculent cells. The underflow from the inclined settler, which was concentrated and enriched with flocculent cells, was recycled to the chemostat. When no recycle was used, the fastergrowing, nonflocculent yeast rapidly overtook the culture. With selective recycle, however, the experiments demonstrated that the slower-growing flocculent yeast could be maintained as the dominant species. A theoretical development is also presented in order to describe the competition between two strains in the bioreactor-settler system. The concept of selective recycle via selective flocculation and sedimentation offers a possible means of maintaining unstable recombinant microorganisms in continuous fermentations.     

15N-labeled 5S RNA. Identification of uridine base pairs in Escherichia coli 5S RNA by 1H-15N multiple quantum NMR

Escherichia coli 5S RNA labeled with 15N at N3 of the uridines was isolated from the S phi-187 uracil auxotroph grown on a minimal medium supplemented with [3-15N]uracil. 1H-15N multiple quantum filtered and 2D chemical shift correlated spectra gave resonances for the uridine imino 1H-15N units whose protons were exchanging slowly with solvent. Peaks with 1H/15N shifts at 11.6/154.8, 11.7/155.0, 11.8/155.5, 12.1/155.0, and 12.2/155.0 ppm were assigned to GU interactions. Two labile high-field AU resonances at 12.6/156.8 and 12.8/157.3 ppm typical of AU pairs in a shielded environment at the end of a helix were seen. Intense AU signals were also found at 13.4/158.5 and 13.6/159.2 ppm where 1H-15N units in normal Watson-Crick pairs resonate. 1H resonances at 10.6 and 13.8 ppm were too weak, presumably because of exchange with water, to give peaks in chemical shift correlated spectra. 1H chemical shifts suggest that the resonance at 13.8 ppm represents a labile AU pair, while the resonance at 10.6 ppm is typical of a tertiary interaction between U and a tightly bound water or a phosphate residue. The NMR data are consistent with proposed secondary structures for 5S RNA.     

Differential survival of chaparral seedlings during the first summer drought after wildfire

Summary Big Pod Ceanothus (Ceanothus megacarpus) is an obligate seeder after fire; Laurel Sumac (Rhus laurina) is primarily a resprouter after fire. Both species commonly occur together in mixed stands and are dominant members of the coastal chaparral of southern California. We compared the mean survival of post-fire seedlings of each species during the first summer drought after fire and found C. megacarpus to have a mean survival of 54% while R. laurina had a mean survival of only 0.1%. Rooting dephs were similar between species but predawn water potentials and leaf temperatures were higher for R. laurina seedlings. Leaf temperatures for R. laurina reached a mean value of 46.8° C on hot, summer days, about 5° C higher than seedlings of C. megacarpus. By the end of the first growing season, 92% of all C. megacarpus seedlings had suffered herbivory compared to only 17% of all R. laurina seedlings. Herbivory did not appear to be the immediate cause of seedling mortality. Transect data indicated that full recovery of prefire species composition and density at our study site was likely but the mode of recovery was different for the species examined. R. laurina recovered primarily by sprouting, C. megacarpus totally by seedling establishment and a third species, Adenostoma fasciculatum (chamise), by a combination of sprouting and seedling establishment. We attribute the higher mortality of R. laurina seedlings to the greater sensitivity of its tissue to water stress. It may be that differential survival of shrub seedlings and differential modes of reestablishment after fire play an important role in maintaining species diversity in the chaparral communities of coastal, southern California.     

Cytoplasmic delivery of ribozymes leads to efficient reduction in alpha-lactalbumin mRNA levels in C127I mouse cells.

Ribozymes targeted to five sites along the alpha-lactalbumin (alpha-lac) mRNA were delivered to the cytoplasm of mouse C127I mammary cells using the T7-vaccinia virus delivery system and the amount of alpha-lac mRNA was monitored 24-48 h post-transfection. Three target sites were selected in the alpha-lac coding region (nucleotides 15, 145 and 361) and two were located in the 3' non-coding region (nucleotides 442 and 694). Acting in trans and at a target:ribozyme ratio of 1:1000, ribozymes targeting sites 361 and 694 reduced alpha-lac mRNA by > 80%; another two ribozymes (targeting nucleotides 442 and 145) reduced mRNA levels by 80 and 60% respectively; the fifth ribozyme (targeting nucleotide 15, near the AUG) was largely ineffective. The kinetic activity (kcat) of each ribozyme in vitro was somewhat predictive of the activity of the two ribozymes that targeted nucleotides 361 and 694, but was not predictive of the in vivo activity of the other three ribozymes. Down-regulation of the intracellular levels of alpha-lac paralleled the ribozyme-dependent reduction achieved for mRNA. For site 442, the reduction in both mRNA and protein was attributed to the catalytic activity of the ribozyme rather than to the antisense effects of the flanking arms, because delivery of an engineered (catalytically-inactive) variant had no effect on mRNA levels and a minimal effect on the level of alpha-lac present in the cell.