(M)CCG METHYLATION IN ANGIOSPERMS

Two different methods were used to investigate the abundance of cytosi ne methylation at the outer (5') position in 5'-CCG-3' trinucleotides in angiosperm genomes. Mspl is unable to cut its target site if the ou ter cytosine is methylated (5'-(m)CCGG-3'). Using Mspl restriction ana lysis, it was shown that 5'-(m)CCG-3' is present in all angiosperm gen omes examined, and that the amount of cytosine methylation at this sit e varies between species. Subsequently, direct measurements were made of the amount of methylation at both cytosines in a subset of 5'-CCG-3 ' trinucleotides in the Arabidopsis thaliana genome. Based upon these analyses, it was estimated that approximately 20-30% of 5'-CCG-3' trin ucleotides in A. thaliana are methylated at the outer cytosine. Approx imately 20% of the 5'-CCG-3' trinucleotides contain li-methyl-cytosine at the inner cytosine position, which corresponds to a previous deter mination of 5'-(m)CG-3' methylation in A. thaliana. The implications o f 5'-(m)CCG-3' methylation are discussed.