Construction of plasmids which lead to overproduction of yeast PHR1 photolyase in Saccharomyces cerevisiae and Escherichia coli.

The PHR1 gene of Saccharomyces cerevisiae encodes a DNA photolyase which is normally present in fewer than 300 copies per cell. We have constructed plasmids in which PHR1 expression in yeast and Escherichia coli is under the control of strong, inducible promoters thereby leading to the regulated overproduction of biologically active photolyase. Under inducing conditions, E. coli cells carrying the tac-PHR1 plasmid pCB1241 accumulate up to 8% of total cellular protein as yeast photolyase; similarly, the GAL10-PHR1 fusion plasmid pGBS107 directs the synthesis of at least 1800-2400 molecules of photolyase per log-phase yeast cell. In both plasmids translation begins at the first ATG in the PHR1 open reading frame (ORF). Constructs in which translation initiates at the second or third ATG fail to complement yeast and E. coli phr1 mutations, indicating that the first ATG in the PHR1 ORF is the translational start site in vivo and that all or part of the N-terminal 78 amino acids are required for activity.