5-Azacytidine inhibits DNA synthesis and to a lesser proportion RNA synthesis in 
S. antibioticus. The biosynthesis of proteins is not affected. The main 
inhibitory effect of 5-azacytidine on DNA and RNA synthesis is probably caused 
by its incorporation into newly synthesized DNA or RNA and the formation of 
covalent complexes between cytosine-specific methyltransferases and the modified 
DNA or RNA templates. To analyze whether such effects could occur at the oriC 
region of S. antibioticus we analyzed the methylation status of this region 
using the bisulphite assisted genomic sequencing method. One of the cytosine 
residues found to be partially methylated was contained within an unique NaeI 
sequence (GCCGGC) in oriC. Subsequent analysis shows chromosomal DNA from S. 
antibioticus to be resistant to R.NaeI restriction indicating that this strain 
contains a NaeI-specific cytosine C5-methyltransferase activity. Following 
5-azacytidine treatment the NaeI site within the oriC region becomes partially 
demethylated. Our results suggest that some of the 5-azacytidine effects on DNA 
and RNA synthesis might indeed be related to the complex formation and 
inhibition of a cytosine-specific DNA methyltransferase.