The chemical industry relies on simpler molecules to build complex compounds, which are used in a variety of applications. Among these organic compounds, ethylene is the building block with the highest demand, being used to manufacture "everyday" products such as polyethylene terephthalate (PET bottles), polyester, anti­freeze, and others (Ungerer et al., 2012). Ethylene (or ethene) is the second simplest unsaturated hydrocarbon, it consists of two carbons, with a double bond, and four hydrogens (H2C=CH2). It is one of the products of pyrolysis, and has been used as a fuel since the early nineteenth century as one of constituents of the gaseous fuel in gas lamps. Ethylene is the most important com­pound in the chemical industry in terms of market volume, it has a heat of combustion higher than that of gasoline or diesel and can be used as a transportation fuel or to produce electric energy in stationary plants. Currently, ethylene is a petroleum derivative produced through steam cracking. It reached a production of 100 million metric tones in 2005, accounting for 30% of all petrochemical commodities (McCoy et al., 2006; Saini and Sigman, 2012). The fluctuation in crude oil prices over the last few years (EIADOE, 2012), the imminent threat of peak oil (Nashawi et al., 2010), and the exis­tence of biological pathways for its production coupled with the ease of harvesting a gas like ethylene, make this chemical a good target for the development of a sus­tainable biological production system.

The most common occurrence of ethylene in nature is as a hormone found in vascular plants, where it is asso­ciated with many effects such as defoliation, responses to temperature stress, mechanical injury, and for promoting fruit ripening (Abeles, 1972). In addition to vascular plants, many other plants and algae have been shown to be able to produce ethylene and even if it not found in animals, this gaseous hormone has been shown to induce regulatory responses in inverte­brate and mammalian cells (Perovic et al., 2001).

The most common biosynthetic pathway for ethylene production is the Yang cycle that occurs in plants, where it is produced from methionine in a three-step reaction, having S-adenosylmethionine (AdoMet) and 1-aminocyclopropane-1-carboxylic acid (ACC) as precursors. However, the cellular response to this hormone occurs at very low concentrations, a characteristic that, together with the fast and easy diffusion of this gas into plant tissues, makes the con­version of AdoMet to ACC, catalyzed by ACC syn­thase, and from ACC to ethylene (ACC oxidase) a tightly regulated process. Both enzymes are multigenic with differential regulation through distinct promoters and operators for groups of genes of the same enzyme (Nakatsuka et al., 1998). The methionine used in this pathway is recycled through the Yang cycle (Taiz and Zeiger, 2002; Wang et al., 2002).