The invention is directed to novel promoters or mutants thereof from Chlorella virus DNA methyltransferase genes. A Chlorella virus gene promoter is operably linked to a first and/or second DNA sequence encoding a gene that is different from the Chlorella virus gene to form an expression cassette. An expression cassette can be introduced into prokaryotic and/or eukaryotic cells and can provide for a high level of expression of the gene encoded by the first and/or second DNA sequence. The invention also provides a method for screening other Chlorella virus genes for promoters that can function to express a heterologous gene in prokaryotic and/or eukaryotic hosts.