The corrected nucleotide sequences of the TaqI restriction and modification enzymes reveal a thirteen-codon overlap

The nucleotide sequence of the genes encoding methyltransferase Taq I (M·T aq I) and restriction endonuclease Taq I (R· Taq I) with the recognition sequence, TCGA, were analyzed in clones isolated from independent libraries. The genes, originally reported as 363 and 236 codons long [Slatko et al., Nucleic Acids Res. 15 (1987)9781-9796] were redetermined as 421 and 263 codons long, respectively. The C terminus of the taqIM gene overlaps the N terminus of the taqIR gene by 13 codons, as observed with the isoschizomeric Tth HB8I restriction-modification system [Barany et al., Gene 112 (1992) 13–20]. Removal of the overlapping codons did not interfere with in vivo M· Taq I activity. We postulate the overlap plays a role in regulating taqIR expression.