Abstract
In cyanobacteria, alkanes are synthesized from a fatty acyl-ACP by two enzymes, acyl–acyl carrier protein reductase and aldehyde deformylating oxygenase. Despite the great interest in the exploitation for biofuel production, nothing is known about the transcriptional organization of their genes or the physiological function of alkane synthesis. The comparison of 115 microarray datasets indicates the relatively constitutive expression of aar and ado genes. The analysis of 181 available genomes showed that in 90% of the genomes both genes are present, likely indicating their physiological relevance. In 61% of them they cluster together with genes encoding acetyl-CoA carboxyl transferase and a short-chain dehydrogenase, strengthening the link to fatty acid metabolism and in 76% of the genomes they are located in tandem, suggesting constraints on the gene arrangement. However, contrary to the expectations for an operon, we found in Synechocystis sp. PCC 6803 specific promoters for the two genes, sll0208 (ado) and sll0209 (aar), which give rise to monocistronic transcripts. Moreover, the upstream located ado gene is driven by a proximal as well as a second, distal, promoter, from which a third transcript, the ~160 nt sRNA SyR9 is transcribed. Thus, the transcriptional organization of the alkane biosynthesis genes in Synechocystis sp. PCC 6803 is of substantial complexity. We verified all three promoters to function independently from each other and show a similar promoter arrangement also in the more distant Nodularia spumigena, Trichodesmium erythraeum, Anabaena sp. PCC 7120, Prochlorococcus MIT9313, and MED4. The presence of separate regulatory elements and the dominance of monocistronic mRNAs suggest the possible autonomous regulation of ado and aar. The complex transcriptional organization of the alkane synthesis gene cluster has possible metabolic implications and should be considered when manipulating the expression of these genes in cyanobacteria.
Highlights
The production of liquid fuels and a variety of chemicals indispensable for daily life depends on fossil resources
A gene encoding a GTP cyclohydrolase I (EC 3.5.4.16), which is involved in folate biosynthesis, was found downstream of the other four
Since the discovery of the cyanobacterial alkane biosynthesis pathway, the major scientific focus has been on the enzymatic properties of aldehyde deformylating oxygenase (ADO) and acyl carrier protein reductase (AAR), as well as their potential for biofuel production (Schirmer et al, 2010; Li et al, 2011, 2012; Zhang et al, 2013)
Summary
The production of liquid fuels and a variety of chemicals indispensable for daily life depends on fossil resources. Alkanes are synthesized from intermediates of the fatty acid metabolism (Figure S1 in Supplementary Material) by two enzymes: acyl–acyl carrier protein reductase (AAR) and aldehyde deformylating oxygenase (ADO) (Schirmer et al, 2010; Li et al, 2011, 2012; Zhang et al, 2013). These enzymes are encoded by the two adjacent genes sll0208 (ado) and sll0209 (aar) in the model strain Synechocystis sp. Orthologs of these genes have been found so far only in cyanobacteria, suggesting the possible existence of a link to photoautotrophic life style i.e., oxygenic photosynthesis, but the functional relevance of cyanobacterial alkane biosynthesis in vivo has remained enigmatic far
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