Author(s)

Pietro Volpe^1,2*

¹Department of Biology, University of Rome “Tor Vergata”, Rome, Italy.
²National Institute of Biostructures and Biosystems, Rome, Italy.

The pathways leading to synthesis and post-synthetic modification of DNA employed methionine as donor of atoms: the carbon that came from its –CH3 served for DNA replication and repair either in bacteria or humans; its entire –CH3 served instead for building N6-methyladenine and 5-methylcytosine on bacterial DNA and 5-methylcytosine alone on human DNA. In humans, although a slight extra-S asymmetric methylation appeared de novo yielding on parental DNA 5’-m5CpC-3’/ 3’-GpG-5’, 5’-m5CpT-3’/3’-GpA-5’ and 5’-m5CpA-3’/3’-GpT-5’ monomethylated dinucleotide pairs, a heavy symmetric methylation involved in S semiconservatively newly made DNA to guarantee genetic maintenance of –CH3 in 5’-m5CpG-3’/3’-Gpm5C-5’ dimethylated dinucleotide pairs. In this framework, an inverse correlation was found between bulk genomic DNA methylation occurring in S and bulk polyA-containing pre-mRNA transcription taking place in G1 and G₂. Thus, probes of 1 × 10⁶ Daltons (constructed using sheared by sonication newly made methylated DNA filaments) revealed a modular organization in genes: after the hypermethylated promoter, they exhibited an alternation of unmethylated coding and methylated uncoding sequences. This encouraged the search for a language that genes regulated by methylation should have in common. An initial deciphering of restriction minimaps with hypomethylatable exons vs. hypermethylatable promoters and introns was improved when the bisulfite technique allowed a direct sequencing of m5C. In lymphocytes, where the transglutaminase gene is inactive, its promoter exhibited two fully methylated CpG-rich domains at 5’ and one fully unmethylated CpG-rich domain at 3’, including the site +1 and a 5’-UTR. At variance, in HUVEC cells, where the transglutaminase gene is active, in the first CpG-rich domain of promoter few doublets lost their –CH₃. Such an inverse correlation suggested new hypotheses especially in connection with repair-modification: UV radiation would cause demethylation in given loci of a promoter by chance, whilst even a partial demethylation in this promoter would be able to resume a previously silent pre-mRNA transcription.

Coding vs. Non-Coding Pre-Messenger RNA Regions, Exons and Introns, Multigenic and Monogenic Transcriptional Units, Regulation of Gene Expression, Repair-Modification

Volpe, P. (2015) The Unexpected Existence of Coding and Non-Coding Fragments along the Eukaryotic Gene. Advances in Biological Chemistry, 5, 98-125. doi: 10.4236/abc.2015.52009.