The effect of E. coli host strain on the consensus sequence of regions of the human L1 transposon

We have used highly methylation tolerant host strains to clone hyper- and hypo-methylated genomic elements from different regions of the same family of long interspersed repetitive elements from human DNA, specifically the 1.8 kilobase (kb) and 1.2kb KpnI fragments from members of the L1 family of transposable elements in which respectively some 18% and 2.7% of cytosines are methylated in vivo in human spleen DNA. The consensus of the DNA sequences of the ends of 13 clones from the hypomethylated region of human L1 agreed exactly with the consensus derived previously from clones made using conventional host strains. However the sequences of 18 of our clones from the 5' end of the hypermethylated region differed significantly from the sequences of clones made using conventional hosts (P less than 0.0001). The 5' region of the 1.8kb L1 region is a CpG island which, in human somatic tissue, appears to be maintained in a highly methylated state, including methylation at sites other than CpG dinucleotides. The consensus sequence of this region also has features suggestive of a previously unrecognized open reading frame.