Difference between revisions of "Part:BBa K754000"

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S. elongatus PCC7942 psbAI promoter

Our part is the Synechococcus elongatus promoter for gene psbAI. This promoter works on a constitutive way, althought its activity can be enhanced or decreased in the presence or absence of light (in nature it works regulated in a circadian cicle). This part is suitable for building light-induced devices.

Sequence and Features

Usage and Biology

The psbAI gene of the cyanobacterium Synechococcus elongatus PCC 7942 is one of three psbA genes that encode a critical photosystem II reaction center protein, D1. Regulation of the gene family in response to changes in the light environment is complex, occurs at transcriptional and posttranscriptional levels, and results in an interchange of two different forms of D1 in the membrane. Expression of psbAI is downregulated under high-intensity light (high light) in contrast to induction of the other two family members.

Cyanobacteria, as well as algae and higher plants, carry out oxygenic photosynthesis, which requires multiprotein complexes that driven by solar energy produce reducing power (NADPH) and chemical energy (ATP). In this system water is the source of electrons in reducing C02 to various organic compounds. The PSII is involved in the water oxidation reaction and the release of oxygen and its core is composed of two critical proteins D1 and D2 (figure 1), which coordinate the cofactors of light-driven charge separation (Andersson and Styring, 1991). Due to the strong oxidative chemistry of the PSII, the D1 protein is subjected to constant photooxidation stress and therefore requires regular replacement to guarantee a steady-state level of D1 protein under different environmental conditions. Under low light growth, the rate of replenishment is 5h, while under intense illumination, the protein is replaced every 20 minutes (Tyystjärvi et al. 1994). In cyanobacteria the three psbA genes that encode the D1 protein are under strict regulation to guarantee the proper functioning of the PSII. In Synechococcus elongatus PCC7942 this three genes encode two distinct D1 protein isoforms: D1:1 being encoded by psbAI and D1:2 by psbAII and psbAIII (Golden et al. 1986).

Functional Parameters

One of the most crucial determinants of gene expression in cyanobacteria is the initiation of transcription, where several sigma factors are involved in promoter recognition (Mulo et al. 2009). The psbAI promoter has characteristic -35 spaced elements from the E. coli σ70 promoter, but has an atypical -10bp element TCTCCT (Golden et al. 1986) (figure 3), which entails that this promoter doesn't work in E. coli (Schaefer and Golden, 1989) making it difficult to characterize it properly. The smallest psbAI functional promoter region comprises nucleotides -54 to +1, and one or more proteins bind specifically to the psbAI upstream region stimulating, rather than inactivating the transcription (+1 to + 43) (Nair et al. 2001), unlike typical σ70 promoters. A segment of approximately 20bp of the consensus -35 element has been shown to be implicated in both, promoter activation per se and light-responsive expression, this region is characterized by being AT-rich (Nair et al. 2001).