Increasing atmospheric concentrations of methane have led scientists to examine its sources of origin. Ruminant livestock can produce 250 to 500 L of methane per day. This level of production results in estimates of the contribution by cattle to global warming that may occur in the next 50 to 100 yr to be a little less than 2%. Many factors influence methane emissions from cattle and include the following: level of feed intake, type of carbohydrate in the diet, feed processing, addition of lipids or ionophores to the diet, and alterations in the ruminal microflora. Manipulation of these factors can reduce methane emissions from cattle. Many techniques exist to quantify methane emissions from individual or groups of animals. Enclosure techniques are precise but require trained animals and may limit animal movement. Isotopic and nonisotopic tracer techniques may also be used effectively. Prediction equations based on fermentation balance or feed characteristics have been used to estimate methane production. These equations are useful, but the assumptions and conditions that must be met for each equation limit their ability to accurately predict methane production. Methane production from groups of animals can be measured by mass balance, micrometeorological, or tracer methods. These techniques can measure methane emissions from animals in either indoor or outdoor enclosures. Use of these techniques and knowledge of the factors that impact methane production can result in the development of mitigation strategies to reduce methane losses by cattle. Implementation of these strategies should result in enhanced animal productivity and decreased contributions by cattle to the atmospheric methane budget.

A single copy of bacteriophage T7 DNA polymerase and DNA helicase advance the replication fork with a processivity greater than 17,000 nucleotides. Nonetheless, the polymerase transiently dissociates from the DNA without leaving the replisome. Ensemble and single-molecule techniques demonstrate that this dynamic processivity is made possible by two modes of DNA polymerase-helicase interaction. During DNA synthesis the polymerase and the helicase interact at a high-affinity site. In this polymerizing mode, the polymerase dissociates from the DNA approximately every 5000 bases. The polymerase, however, remains bound to the helicase via an electrostatic binding mode that involves the acidic C-terminal tail of the helicase and a basic region in the polymerase to which the processivity factor also binds. The polymerase transfers via the electrostatic interaction around the hexameric helicase in search of the primer-template.

Liver and gastrointestinal tract weights (ingesta- and adipose-free) appear to increase or decrease in direct proportion to dietary intake within and across physiological stages of maintenance, growth, fattening or lactation. Liver and gut mass increase approximately 15 and 30 g per unit of liveweight raised to the 0.75 power (Wt0.75) for each multiple of 500 kJ/Wt0.75 [approximately 1 x maintenance (M)] increase in metabolizable energy (ME) intake, with linearity indicated up to the highest recorded level (4.5 x M). Extrapolation from in vivo arteriovenous O2 measurements across splanchnic tissues and from the previously cited weight information indicates that liver and gut tissue oxidize approximately 3.5 and 1.0 kJ of ME/g of fresh tissue daily, in contrast to whole-animal rates of 0.1 kJ/g. Thus, energy use by the relatively small amount of liver and gut accounts for 45 to 50% of whole-animal heat energy. On a differential basis, increases in energy use by these tissues appear to account for up to 70% of the heat increment of ME use above maintenance.

The carcinogen 2-acetylaminofluorene forms two major DNA adducts: N-(2-deoxyguanosin-8-yl)-2-acetylaminofluorene (dG-AAF) and its deacetylated derivative, N-(2-deoxyguanosin-8-yl)-2-aminofluorene (dG-AF). Although the dG-AAF and dG-AF adducts are distinguished only by the presence or absence of an acetyl group, they have profoundly different effects on DNA replication. dG-AAF poses a strong block to DNA synthesis and primarily induces frameshift mutations in bacteria, resulting in the loss of one or two nucleotides during replication past the lesion. dG-AF is less toxic and more easily bypassed by DNA polymerases, albeit with an increased frequency of misincorporation opposite the lesion, primarily resulting in G 3 T transversions. We present three crystal structures of bacteriophage T7 DNA polymerase replication complexes, one with dG-AAF in the templating position and two others with dG-AF in the templating position. Our crystallographic data suggest why a dG-AAF adduct blocks replication more strongly than does a dG-AF adduct and provide a possible explanation for frameshift mutagenesis during replication bypass of a dG-AAF adduct. The dG-AAF nucleoside adopts a syn conformation that facilitates the intercalation of its fluorene ring into a hydrophobic pocket on the surface of the fingers subdomain and locks the fingers in an open, inactive conformation. In contrast, the dG-AF base at the templating position is not well defined by the electron density, consistent with weak binding to the polymerase and a possible interchange of this adduct between the syn and anti conformations. N umerous carcinogenic aromatic amines, including a variety of environmental and dietary carcinogens and heterocyclic aromatic amines present in tobacco smoke condensate, are known to react with DNA to form adducts at the C8 position of guanine (1). 2-Acetylaminofluorene (AAF) is the best-studied example of this class of carcinogen (2). Originally developed as a pesticide, toxicity tests showed that this compound and related derivatives are potent liver carcinogens (3). Thus, the compound was never introduced as a pesticide. Instead, AAF has become a model compound for the study of the mutagenic and carcinogenic effects of aromatic amines (4).Metabolic activation of AAF in vivo generates intermediates that form two related adducts bound to the C8 position of guanine DNA: the N-(2Ј-deoxyguanosin-8-yl)-AAF (dG-AAF) adduct and the corresponding deacetylated N-(2Ј-deoxyguanosin-8-yl)-2-aminofluorene (dG-AF) derivative (Fig. 1) (3). The mutagenic consequences of these adducts are quite distinct in Escherichia coli. The dG-AF adduct predominately produces randomly distributed base-substitution mutations (5, 6), whereas the dG-AAF adduct results in frameshift mutations that frequently target specific repetitive sequences (4, 7-9). In vitro studies using templates modified with either a dG-AF or dG-AAF adduct have shown that the 2-aminofluorene (AF) adduct is bypassed much more readily than the corresponding AAF adduct by a variety of DNA pol...