The genomic sequence of the archaeon Methanosarcina mazei has been analyzed by 
the Z curve method. The Z curve is a three-dimensional curve that uniquely 
represents the given DNA sequence. The three-dimensional Z curve and its x and y 
components for the genome of M. mazei show a sharp peak and relatively broad 
peak, respectively. The cdc6 gene is located exactly at the position of the 
sharp peak. Based on the known behavior of the Z curves for the archaea whose 
replication origins have been identified, we hypothesize that the replication 
origin and termination sites correspond to the positions of the sharp peak and 
broad peak, respectively. We have located an intergenic region that is between 
the cdc6 gene (MM1314) and the gene for an adjacent protein (MM1315), which 
shows strong characteristics of the known replication origins. This region is 
highly rich in AT and contains multiple copies of consecutive repeats. Our 
results strongly suggest that the single replication origin of M. mazei is 
situated at the intergenic region between the cdc6 gene and the gene for the 
adjacent protein, from 1,564,657 to 1,566,241 bp of the genome.