NOVELTY - Improving efficiency of carbon dioxide (CO2) fixation in an organism having a Calvin-Benson-Bassham (CBB) cycle, comprises genetically modifying the organism with the CBB cycle to produce or overexpress a first enzyme with a phosphoketolase activity, and to produce or overexpress a second enzyme with a phosphoribulokinase activity. The first enzyme utilizes an intermediate of the CBB pathway as a substrate and generates a first acetyl phosphate product.    USE - The methods are useful for improving efficiency of CO2 fixation in an organism having CBB cycle; for reducing accumulation of PEP in CBB dependent microorganism under nitrogen limitation condition; and for producing value added product in microorganism having CBB cycle, where the value added product is selected from alcohol, fuel, plastic polymer, and monomer suitable for plastic polymer synthesis; or the value added product is n-butanol, isobutanol, an alkene, or biodiesel (all claimed).    ADVANTAGE - Methods and compositions are provided that permit efficient carbon fixation by autotrophic organisms under conditions that also permit efficient production of value added materials without imposing undue metabolic burden and additional catalytic activities onto a cell. Moreover, metabolically engineered cells are provided that can produce value products at a high rate under nitrogen-limiting or nitrogen depletion conditions without feedback inhibition by CBB cycle metabolite that accumulates under such conditions.    DETAILED DESCRIPTION - Improving efficiency of carbon dioxide (CO2) fixation in an organism having a Calvin-Benson-Bassham (CBB) cycle, comprises genetically modifying the organism with the CBB cycle to produce or overexpress a first enzyme with a phosphoketolase activity, and to produce or overexpress a second enzyme with a phosphoribulokinase activity. The first enzyme utilizes an intermediate of the CBB pathway as a substrate and generates a first acetyl phosphate product. The phosphoribulokinase activity is produced or overexpressed in an amount to achieve a phosphoribulokinase activity level that is higher than the native phosphoribulokinase activity level of the organism. Production of phosphoenolpyruvate (PEP) in the genetically modified organism is decreased relative to that of a non-modified organism of the same species. The first acetyl phosphate product is converted in the organism to acetyl-coenzyme A (CoA). INDEPENDENT CLAIMS are:    (1) a metabolically engineered cell having a native CBB cycle, comprising a recombinant nucleic acid comprising a nucleic acid sequence encoding a first enzyme with a phosphoketolase activity and a second enzyme with a phosphoribulokinase activity; where the first enzyme utilizes an intermediate of the CBB pathway as a substrate and generates a first acetyl phosphate product; and where the phosphoribulokinase activity is produced or overexpressed in an amount that avoids depletion of ribulose-5-phosphate in the CBB cycle;    (2) a method of reducing accumulation of PEP in a CBB dependent microorganism under nitrogen limitation condition, comprising genetically modifying the microorganism to produce or overexpress a first enzyme with a phosphoketolase activity to generate acetylphosphate from an intermediate of the CBB cycle; genetically modifying the organism with the CBB cycle to produce or overexpress a second enzyme with a phosphoribulokinase activity, where the phosphoribulokinase activity is produced or overexpressed in an amount to achieve a phosphoribulokinase activity level that is higher than the native phosphoribulokinase activity level of the organism; and using the microorganism to withdraw the acetylphosphate from the CBB cycle via conversion of the acetylphosphate to acetyl-CoA to so reduce availability of glyceraldehyde-3 -phosphate for PEP formation; and    (3) a method for producing a value added product in a microorganism having a CBB cycle, comprising providing a genetically modified organism having a recombinant first enzyme with a phosphoketolase activity, and where the genetically modified organism overexpresses a second enzyme in the CBB cycle having an equilibrium constant of at least 1000; where the genetically modified organism produces one molecule of acetyl-CoA through fixation of two molecules of CO2 in the CBB cycle; culturing the genetically modified organism, optionally under low nitrogen conditions, while supplying a source of CO2; where the genetically modified organism uses the acetyl-CoA to produce a value added product selected from an alcohol, a fuel, a plastic polymer, and a monomer suitable for plastic polymer synthesis.