We have designed a bacterial system that is switched between different states by red light. The system consists of a synthetic sensor kinase that allows a lawn of bacteria to function as a biological film, such that the projection of a pattern of light on to the bacteria produces a high-definition (about 100 megapixels per square inch), two-dimensional chemical image. This spatial control of bacterial gene expression could be used to 'print' complex biological materials, for example, and to investigate signalling pathways through precise spatial and temporal control of their phosphorylation steps.

Cells transmit and receive information via signalling pathways. A number of studies have revealed that information is encoded in the temporal dynamics of these pathways and has highlighted how pathway architecture can influence the propagation of signals in time and space. The functional properties of pathway architecture can also be exploited by synthetic biologists to enable precise control of cellular physiology. Here, we characterised the response of a bacterial light-responsive, two-component system to oscillating signals of varying frequencies. We found that the system acted as a low-pass filter, able to respond to low-frequency oscillations and unable to respond to high-frequency oscillations. We then demonstrate that the low-pass filtering behavior can be exploited to enable precise control of gene expression using a strategy termed pulse width modulation (PWM). PWM is a common strategy used in electronics for information encoding that converts a series of digital input signals to an analog response. We further show how the PWM strategy extends the utility of bacterial optogenetic control, allowing the fine-tuning of expression levels, programming of temporal dynamics, and control of microbial physiology via manipulation of a metabolic enzyme.

Natural and engineered RNA 'parts' can perform a variety of functions, including hybridizing to targets, binding ligands and undergoing programmed conformational changes, and catalyzing reactions. These RNA parts can in turn be assembled into synthetic genetic circuits that regulate gene expression by acting either in cis or in trans on mRNAs. As more parts are discovered and engineered, it should be increasingly possible to create synthetic RNA circuits that are able to carry out complex logical operations in cells, either superimposed on or autonomous to extant gene regulation.

Efforts to reduce tropospheric ozone (O 3 ) pollution in the southeastern United States by reducing emissions of volatile organic compounds have had only modest success, and attention is now being drawn to sources of nitrogen oxide (NO x ), which is the other major precursor of photochemical production of O 3 . Emissions of nitric oxide (NO) from the soils of this region are poorly known. In this study, we adapt the CASA model of terrestrial-ecosystem productivity and trace-gas emissions to make spatially gridded, monthly estimates of NO emissions from soils for a nine-state region of the southeastern United States. Gridded input data layers to the model include temperature, precipitation, solar surface radiation, soil texture, land cover, fertilizer sales by county, and normalized-difference vegetation index. Total N gas emissions are modeled as proportional to the rates of gross N mineralization and fertilizer N input. The relative proportional emissions of NO, N 2 O, and N 2 are determined by soil water content. The model estimates 126 ϫ 10 9 g NO-N/yr emitted from soils for the entire nine-state area. Agricultural soils emit two-thirds of the total but cover only ϳ17% of the land area. Highest NO emissions occur along the cultivated Mississippi River corridor, southern Alabama-Georgia, and the Carolina coastal plain. Simulated emissions fall within the range of commonly reported values for temperate forests and cultivated fields, and the regional estimates are probably accurate within a factor of 2. Compared to NO x emissions from industrial, transportation, and utility sectors, soil emissions are ϳ10% of total regional emissions. However, the soil source could be a significant fraction of local NO x in rural agricultural areas, where emissions from fertilized fields usually range from 300 to 900 mg NO-N·m Ϫ2 ·yr Ϫ1 . Fertilizer management could be an effective O 3 -abatement strategy in some rural agricultural areas, although this possibility and general uncertainties in the soil source of NO should not be grounds to delay other O 3 -abatement strategies for the much larger, nonsoil NO x sources in the region.

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