Another key feature of the invention is the application of a genetic switch to control the expression of the designer ethanol-producing pathway(s), as illustrated in Fig. 2. This switchability is accomplished through the use of an externally inducible pro­moter so that the designer transgenes are inducibly expressed under certain specific inducing conditions (Fig. 6a) . Preferably, the promoter employed to control the expression of designer genes in a host is originated from the host itself or a closely related organism. The activities and inducibility of a promoter in a host cell can be tested by placing the promoter in front of a reported gene, introducing this reporter construct into the host tissue or cells by any of the known DNA delivery techniques, and assessing the expression of the reporter gene.

In a preferred embodiment, the inducible promoter used to control the expression of designer genes is a promoter that is inducible by anaerobiosis, i. e., active under anaerobic conditions but inactive under aerobic conditions. A designer alga/plant organism can perform autotrophic photosynthesis using CO2 as the carbon source under aerobic conditions (Fig. 4a), and when the designer organism culture is grown and ready for photosynthetic ethanol production, anaerobic conditions will be applied to turn on the promoter and the designer genes (Fig. 4b).

A number of promoters that become active under anaerobic conditions are suit­able for use in the present invention. For example, the promoters of the hydrogenase genes (HydA1 (Hyd1) and HydA2, GenBank accession number: AJ308413, AF289201, AY090770) of C. reinhardtii. which is active under anaerobic condi­tions but inactive under aerobic conditions, can be used as an effective genetic switch to control the expression of the designer genes in a host alga, such as C. reinhardtii. In fact, Chlamydomonas cells contain several nuclear genes that are coordinately induced under anaerobic conditions. These include the hydrogenase structural gene itself (Hyd1), the Cyc6 gene encoding the apoprotein of Cytochrome C6, and the Cpxl gene encoding coprogen oxidase. The regulatory regions for the latter two have been well characterized, and a region of about 100 bp proves sufficient to confer regulation by anaerobiosis in synthetic gene constructs [ 5]. Although the above inducible algal promoters may be suitable for use in other plant hosts, especially in plants closely related to algae, the promoters of the homologous genes from these other plants, including higher plants, can be obtained and employed to control the expression of designer genes in those plants.

In another embodiment, the inducible promoter used in the present invention is an algal nitrate reductase (Nia1) promoter, which is inducible by growth in a medium containing nitrate and repressed in a nitrate-deficient but ammonium-containing medium [6]. Therefore, the Nia1 (gene accession number AF203033) promoter can be selected for use to control the expression of the designer genes in an alga according to the concentration levels of nitrate in a culture medium. Additional inducible promoters that can also be selected for use in the present invention include, for example, the heat-shock protein promoter HSP70A [7] (accession number: DQ059999, AY456093, M98823), the promoter of CabII-1 gene (accession number M24072), the promoter of Ca1 gene (accession number P20507), and the promoter of Ca2 gene (accession number P24258). Throughout this specification, when refer­ence is made to inducible promoter, such as, for example, any of the inducible pro­moters described above, it includes their analogs, functional derivatives, designer sequences, and combinations thereof. A “functional analog” or “modified designer sequence” in this context refers to a promoter sequence derived or modified (by, e. g., substitution, moderate deletion or addition or modification of nucleotides) based on a native promoter sequence, such as those identified hereinabove, that retains the function of the native promoter sequence.