The structural difference between the two modifications represents the ordered, head-to-tail arrangement of OH groups located in the center of every other Ca2 triangle in the monoclinic low-temperature symmetry and the disordered arrangement of OH groups, where the head-to-tail and tail-to-head arrangements alternate throughout the channel in the hexagonal high-temperature symmetry.
Previous studies have shown that phosphorylation of histone H3 at serine 10 (H3S10P) antagonizes the binding of HP1 to H3K9me, and that the binary phospho-methyl switch between the two modifications is crucial for heterochromatin assembly in humans (Fischle et al., 2003).
In yeast, Rad18 is rate limiting for both mono- and polyubiquitylation of PCNA (Davies et al. 2008), but the conditions that control the activation of Ubc13-Mms2 and Rad5 or determine the balance between the two modifications have not been determined.
However, it remains to be determined whether the deubiquiylation process is involved in competition between these two modification states.
Lastly, we demonstrated that acetylation at K487 and sumoylation at K490 in PML are mutually exclusive, suggesting a negative crosstalk between these two modification [ 116].
Our previous observations of Ser phosphorylation regulating subsequent ubiquitination left the relationship between these two modifications and the enzymes involved unknown [ 15].
In fact, the only fundamental difference between these two modifications is the symmetry of the C-2 chain formed by the respective A-type molecules (I: glide symmetry, II: inversion; see Additional file 1: Fig. S4).
It was also reported that 5-hmC is mostly associated with euchromatin and, whereas 5-mC is under-represented at gene promoters and CpG islands, 5-hmC is enriched in gene bodies and is associated with increased transcriptional levels.,, Most, if not all, 5-hmC in the genome depends on pre-existing 5-mC, and the balance between these two modifications varies depending on the genomic region.
The negative correlation between these two modifications during the cell cycle is consistent with previous findings that H4K20me1 antagonizes H4K16ac [ 47].
