Nucleic Acids Research, 1990, Vol. 18, No. 23 6965-6969
© 1990
Articles |
Absence of methylation at Hpall sites in three human genomic tRNA sequences
Departments of Medicine and Genetics, SK-50, University of Washington Seattle, WA 98195, USA
Received August 13, 1990. Revised November 6, 1990. Accepted November 6, 1990.
It has been known since the development of nearest neighbor analysis that the frequency of the dinucleotide CpG is markedly suppressed in vertebrate DNA (i.e. less than %C x xG). This suppression appears to be heterogeneous since it was shown some years ago that three vertebrate tRNA genes did not exhibit CpG suppression. We have analyzed 13 different human tRNA genes and found that they also do not exhibit CpG suppression. Because CpG suppression has been linked, to some extent at least, to the methylationdeamination process by which a methylated CpG is mutated to TpG, we investigated whether the lack of suppression of CpG in tRNAs could originate from anabsence of methylation.
Three human tRNA genes were selected from Genbank (Lysine, Proline, and Phenylalanine) and examined for methylation at Hpall sites by polymerase chain reaction (PCR) and Southern blot analysis. The observed patterns were consistent with the absence of methylation at the seven Hpall sites analyzed in and around the tRNA genes, and we predict that the remaining CpGs in these genes will be unmethylated. Since GC-rich promoter regions also escape CpG suppression and since they are generally unmethylated, avoidance of methylation may be a general explanation for the absence of CpG suppression in selected regions of vertebrate genomes.