The multisubunit transcription factor TFIID is essential for directing eukaryotic promoter recognition and mediating interactions with activators/cofactors during assembly of the preinitiation complex. Despite its central role in transcription initiation and regulation, structural knowledge of the TFIID complex has so far been largely limited to electron microscopy studies of negatively stained samples. Here, we present a cryo-electron microscopy 3D reconstruction of the large endogenous human TFIID complex. The improved cryopreservation has allowed for a more detailed definition of the structural elements in the complex and for the detection, by an extensive statistical analysis of the data, of a conformational opening and closing of the cavity central to the TFIID architecture. We propose that these density rearrangements in the structure are a likely reflection of the plasticity of the interactions between TFIID and its many partner proteins.

ErratumERRATUM low-external-input and conventional systems to refl ect the updated data set, as well as changing the primary energy input category from grain handling to fertilizer. Th e corrections change the quantitative results reported in the publication, but with the exception of a shift in the relative importance of grain handling and fertilizer, the general pattern of results and conclusions remains the same.My coauthors and I sincerely apologize for the error in the publication.-Michael Cruse ABSTRACT Conventional agriculture production systems in developed countries rely heavily on fossil energy, but emerging uncertainties in fossil fuel supply indicate a need to better understand energy effi ciency in conventional and alternative systems. We used 6 yr of data from a cropping systems experiment conducted in Iowa to compare energy use of a conventionally managed corn (Zea mays L.)soybean [Glycine max (L.) Merr.] system (a 2-yr rotation) with two low-external input (LEI) cropping systems that used more diverse rotations and manure, but substantially lower quantities of synthetic N fertilizer and herbicides. Depending on how fossil energy costs were assigned to manure, the two LEI systems (a 3-yr rotation of corn-soybean-small grain/red clover (Trifolium pratense L.), and a 4-yr rotation of corn-soybean-small grain/alfalfa-alfalfa, Medicago sativa L.) used between 14 and 53% less fossil energy than did the conventional system. In general, the primary category for fossil energy use was fertilizer. Th e conventional 2-yr system used substantially more fossil energy embodied in synthetic fertilizers and pesticides than did the LEI systems. Economic return, harvested crop weight, and potential energy production of the conventional 2-yr and LEI 4-yr systems were similar. Effi ciency ratios, including crop energy output and economic return per unit of fossil energy invested, were signifi cantly higher in the LEI 4-yr rotation than in the conventional system. In coming years, if fossil energy prices rise signifi cantly without concomitant increases in crop value, diversifi ed LEI systems may become preferable to conventional cropping systems and used more widely.

ErratumERRATUM low-external-input and conventional systems to refl ect the updated data set, as well as changing the primary energy input category from grain handling to fertilizer. Th e corrections change the quantitative results reported in the publication, but with the exception of a shift in the relative importance of grain handling and fertilizer, the general pattern of results and conclusions remains the same.My coauthors and I sincerely apologize for the error in the publication. -Michael CruseABSTRACT Conventional agriculture production systems in developed countries rely heavily on fossil energy, but emerging uncertainties in fossil fuel supply indicate a need to better understand energy effi ciency in conventional and alternative systems. We used 6 yr of data from a cropping systems experiment conducted in Iowa to compare energy use of a conventionally managed corn (Zea mays L.)soybean [Glycine max (L.) Merr.] system (a 2-yr rotation) with two low-external input (LEI) cropping systems that used more diverse rotations and manure, but substantially lower quantities of synthetic N fertilizer and herbicides. Depending on how fossil energy costs were assigned to manure, the two LEI systems (a 3-yr rotation of corn-soybean-small grain/red clover (Trifolium pratense L.), and a 4-yr rotation of corn-soybean-small grain/alfalfa-alfalfa, Medicago sativa L.) used between 14 and 53% less fossil energy than did the conventional system. In general, the primary category for fossil energy use was fertilizer. Th e conventional 2-yr system used substantially more fossil energy embodied in synthetic fertilizers and pesticides than did the LEI systems. Economic return, harvested crop weight, and potential energy production of the conventional 2-yr and LEI 4-yr systems were similar. Effi ciency ratios, including crop energy output and economic return per unit of fossil energy invested, were signifi cantly higher in the LEI 4-yr rotation than in the conventional system. In coming years, if fossil energy prices rise signifi cantly without concomitant increases in crop value, diversifi ed LEI systems may become preferable to conventional cropping systems and used more widely.

Plant canopy interception of photosynthetically active radiation (PAR) drives carbon dioxide (CO2), water and energy cycling in the soil-plant-atmosphere system. Quantifying intercepted PAR requires accurate measurements of total incident PAR above canopies and direct beam and diffuse PAR components. While some regional data sets include these data, e.g. from Atmospheric Radiation Measurement (ARM) Program sites, they are not often applicable to local research sites because of the variable nature (spatial and temporal) of environmental variables that influence incoming PAR. Currently available instrumentation that measures diffuse and direct beam radiation separately can be cost prohibitive and require frequent adjustments. Alternatively, generalized empirical relationships that relate atmospheric variables and radiation components can be used but require assumptions that increase the potential for error. Our goal here was to construct and test a cheaper, highly portable instrument alternative that could be used at remote field sites to measure total, diffuse and direct beam PAR for extended time periods without supervision. The apparatus tested here uses a fabricated, solar powered rotating shadowband and other commercially available parts to collect continuous hourly PAR data. Measurements of total incident PAR had nearly a one-to-one relationship with total incident radiation measurements taken at the same research site by an unobstructed point quantum sensor. Additionally, measurements of diffuse PAR compared favorably with modeled estimates from previously published data, but displayed significant differences that were attributed to the important influence of rapidly changing local environmental conditions. The cost of the system is about 50% less than comparable commercially available systems that require periodic, but not continual adjustments. Overall, the data produced using this apparatus indicates that this instrumentation has the potential to support ecological research via a relatively inexpensive method to collect continuous measurements of total, direct beam and diffuse PAR in remote locations.

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