Base ratios and total DNA amounts can vary substantially between and within
higher taxa and genera, and even within species. Gene conversion is one of
several mechanisms that could cause such changes. For base substitutions,
disparity in conversion direction is accompanied by an equivalent disparity
in base ratio at the heterozygous site. Disparity in the direction of gene
conversion at meiosis is common and can be extreme. For transitions (which
give purine [R]/pyrimidine [Y] mispairs) and for transversions giving
unlike R/R and Y/Y mispairs in hybrid DNA, this disparity could give slow
but systematic changes in G + C percentage. For transversions giving like
R/R and Y/Y mispairs, it could change AT/TA and CG/GC ratios. From the
extent of correction direction disparity, one can deduce properties of
repair enzymes, such as the ability (1) to excise preferentially the purine
from one mispair and the pyrimidine from the other for two different R/Y
mispairs from a single heterozygous site and (2) to excise one base
preferentially from unlike R/R or Y/Y mispairs. Frame-shifts usually show
strong disparity in conversion direction, with preferential cutting of the
nonlooped or the looped-out strand of the nonpair in heterozygous h-DNA.
The opposite directions of disparity for frame-shifts and their intragenic
suppressors as Ascobolus suggest that repair enzymes have a strong,
systematic bias as to which strand is cut. The conversion spectra of
mutations induced with different mutagens suggest that the nonlooped strand
is preferentially cut, so that base additions generally convert to mutant
and deletions generally convert to wild-type forms. Especially in
nonfunctional or noncoding DNA, this could cause a general increase in DNA
amounts. Conversion disparity, selection, mutation, and other processes
interact, affecting rates of change in base ratios and total DNA.
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The haemopoietic growth factors interleukin 3 (IL-3) and colony stimulating factor-1 (CSF-1) stimulate the survival and proliferation of murine normal bone-marrow-derived macrophages. To establish whether these growth factors elicit their effects via the hydrolysis of phosphatidylinositol(4,5)bisphosphate [PtdIns(4,5)P2] to form the second messengers inositol (1,4,5)trisphosphate [Ins(1,4,5)P3] and diacylglycerol, macrophages were labelled with tracer quantities of [3H]inositol. Treatment of these cells with either IL-3 or CSF-1 did not alter the levels of PtdIns(4,5)P2 or Ins(1,4,5)P3. However, addition of the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) which does not stimulate proliferation in macrophages caused a marked and rapid increase in the levels of Ins(1,4,5)P3, inositol bisphosphate and inositol monophosphate, and a decrease in the amount of PtdIns(4,5)P2. FMLP also evoked a rapid increase in intracellular cytosolic Ca2+ levels, as measured with quin 2 the Ca2+-sensitive fluorescent probe, whereas IL-3 and CSF-1 had no such effect. These results suggest that FMLP stimulates the hydrolysis of PtdIns(4,5)P2 to form the second messenger Ins(1,4,5)P3 which acts to increase the levels of cytosolic Ca2+, and that IL-3- and CSF-1-stimulated proliferation in macrophages is not associated with the formation of PtdIns(4,5)P2-derived second messengers. 相似文献
The metabolism of the free glucose pool in rumen digesta from sheep fed roughage rations was studied by adding an insignificant quantity of glucose as uniformly labeled (14)C-glucose of high specific activity to in vitro incubation systems. In all experiments wherein only trace quantities of glucose were added to digesta, most of the (14)C-glucose entered acetate. This was true whether label was presented either as a single dose or by continuous addition over a period of 2 hr. Digesta collected at all times after feeding either once daily or at hourly intervals gave similar glucose dissimilation patterns. If, however, a relatively large quantity of carrier glucose was added together with the tracer, the (14)C-acetate: (14)C-propionate ratio was reduced by a factor of about 10. Physical removal of most of the protozoa from digesta generally had little effect on the dissimilation of (14)C-glucose added in tracer amounts, but in one experiment there was a decreased turnover of the free glucose pool and a marked reduction in (14)C entering butyrate. The paucity of (14)C entering propionate when only trace amounts of glucose were added to digesta suggests that this acid was largely formed from substrates whose carbon did not equilibrate with that in free glucose or with that in intermediates of free glucose metabolism. 相似文献
Summary Irradiation of the colI donor itself is required to initiate indirect induction of phage following mating with a lysogenic recipient. Attempts to demonstrate that some other cytoplasmic inducer is responsible for induction have been negative. However the level of transfer of a viable (colicin-producing) colI factor (to a non-lysogenic recipient) is not correlated with the transfer of indirect induction (to a lysogenic recipient) either with respect to relative UV sensivities, or relative kinetics. Transfer of viable colI factor from the irradiated donor is delayed for up to 40 minutes whereas indirect induction is initiated early after contact. There is some lethality and inhibition of division of recipient cells mated with the irradiated donor, these effects similarly being initiated extremely early after cell contact. The question as to whether these effects of mating with an irradiated donor on the recipient follow transfer of inviable colI, or reflect a disturbance of transfer itself remains unresolved. 相似文献
The viability of washed moist cells of Serratia marcescens after storage has been measured in relation to variations in the prior treatment of the cells and in conditions of storage. The factors considered were: (i) water content during storage; (ii) method of arriving at water content (partial drying in vacuum or freeze-drying and addition of water); (iii) presence or absence of air during storage.
Increasingly rapid decay occurs as the water content at which the cells are stored is diminished from above 90% to 20 or 30% (“critical” water content). It occurs in presence or absence of air and it occurs whether the final water content is approached by removal of water from wet cells or by addition of water to freeze-dried cells.
The rate of decay during storage at 20 to 30% water is somewhat diminished by the presence of air (“protective” effect of air).
As the water content is further reduced to less than 10%, the stability of cells stored in a vacuum approaches that of wet cells. In presence of air the reverse is true: the stability decreases until at less than 1% water, the decay rate is about as great as at the “critical” water content (“toxic” effect of air).
Particularly rapid decay of S. marcescens at the “critical” water content has escaped attention in aerosol studies because accurate control of relative humidity (RH) in this region, RH 94 to 99%, is virtually impossible in such studies. On the other hand, values of decay rates referred to measured water contents are quite unreliable in the 20 to 80% RH zone because the corresponding variation of water content is too small to measure reliably. Thus data of the kind reported in this paper cannot be directly compared to the published results of studies of air-borne bacteria, although they are relevant to the practical question of air-borne infection in humid atmospheres.
The herpes simplex virus 1 US11 protein is an RNA-binding regulatory protein that specifically and stably associates with 60S ribosomal subunits and nucleoli and is incorporated into virions. We report that US11/ beta-galactosidase fusion protein expressed in bacteria bound to rRNA from the 60S subunit and not the 40S subunit. This binding reflects the specificity of ribosomal subunit association. Analyses of deletion mutants of the US11 gene showed that specific RNA binding activity, nucleolar localization, and association with 60S ribosomal subunits were found to map to the amino acid sequences of the carboxyl terminus of US11 protein, suggesting that these activities all reflect specific binding of US11 to large subunit rRNA. The carboxyl-terminal half of the protein consists of a regular tripeptide repeat of the sequence RXP and constitutes a completely novel RNA-binding domain. All of the mutant US11 proteins could be incorporated into virus particles, suggesting that the signal for virion incorporation either is at the amino-terminal four amino acids or is redundant in the protein. 相似文献