Fusion constructs as protein over-expression vectors

An expression construct comprising a polynucleotide encoding a fusion protein, said fusion protein comprising a protein of interest to be expressed in cyanobacteria joined to a bacterial neomycin phosphotransferase protein that confers resistance to kanamycin or a chloramphenicol acetyl transferase protein that confers resistance to chloramphenicol;

• wherein the polynucleotide comprises a transgene encoding the protein of interest that is codon-optimized for expression in cyanobacteria and is joined to the 3′ end of a nucleic acid encoding the bacterial neomycin phosphotransferase (NPT) protein or the chloramphenicol acetyltransferase protein.