Auxiliary metabolic genes- Distinct Features of Cyanophage-encoded T-type Phycobiliprotein Lyase {theta}CpeT [Protein Structure and Folding]

Auxiliary metabolic genes (AMG) are commonly found in the genomes of phages that infect cyanobacteria and increase cyanophage′s fitness. AMGs are often homologs of host genes, and also typically related to photosynthesis. For example, the θcpeT gene in the cyanophage P-HM1 encodes a putative phycobiliprotein lyase related to cyanobacterial T-type lyases, which facilitate attachment of linear tetrapyrrole chromophores to Cys-155 of phycobiliprotein β-subunits, suggesting that θCpeT may also help assemble light-harvesting phycobiliproteins during infection. In order to investigate this possibility, we structurally and biochemically characterized recombinant θCpeT. The solved crystal structure of θCpeT at 1.8 Å resolution revealed that the protein adopts a similar fold as the cyanobacterial T-type lyase CpcT from Nostoc sp. PCC7120 but overall is more compact and smaller. θCpeT specifically binds phycoerythrobilin (PEB) in vitro leading to a tight complex that can also be formed in Escherichia coli when it is co-expressed with genes encoding PEB biosynthesis (i.e. ho1 and pebS). The formed θCpeT:PEB complex was very stable as the chromophore was not lost during chromatography and displayed a strong red fluorescence with a fluorescence quantum yield of θF=0.3. This complex was not directly able to transfer PEB to the host phycobiliprotein β-subunit. However, it could assist the host lyase CpeS in its function by providing a pool of readily available PEB, a feature that might be important for fast phycobiliprotein assembly during phage infection.

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