Received September 30, 1999
According to the proposed hypothesis, the memory of a cell about the achieved state of cytodifferentiation is based on the existence of a postulated genetic structure termed here as a "printomere". A printomere is a relatively small linear DNA fragment which is laterally located on the chromosomal body and armed at its termini with peculiar analogs of chromosomal telomeres, which in this case are designated as "acromeres". The printomere locates along its chromosomal original--protoprintomere--and is bound to this chromosomal segment via proteins. The printomere codes for so-called fountain RNAs (fRNAs). Molecules of fRNAs as a part of ribonucleoproteins, or fRNPs, specifically bind to the complementary for them DNA sites, or "fions", that are dispersed nearby many structural genes. fRNP--fion complexes help to open, for a very short time, closed ion channels in the inner nuclear membrane, and this occurs strictly nearby corresponding genes. Dosed and local entry of the specific ions from the perinuclear cistern of the nucleus modifies the local pattern of the chromatin decompaction and modulates the expression level of the corresponding genes. The implied role of the fRNAs was considered in the so-called "fountain theory" (A. M. Olovnikov (1997) Int. J. Dev. Biol., 41: 923-931; A. M. Olovnikov (1999) J. Anti-Aging Medicine, 2: 57-71; A. M. Olovnikov (1999) Advances in Gerontology (St. Petersburg), 3: 54-64). Transcripts (fRNAs) coded by printomeres participate in the creation and maintenance of the specific patterns of decompaction and compaction of chromatin, which are characteristic for corresponding cytodifferentiations. Printomeres of various differentiations differ in their nucleotide sequences. The printomere and its chromosomal original, the protoprintomere, located co-linearly, side by side with it, have their own ori. Their length may vary from several thousands of base pairs to tens of thousands of b.p. Printomere bound by its arms to the chromosomal DNA with chromatin proteins is able to pass over the replicative forks during printomere replication and replication of the chromosome. That is why any printomere can be stably retained on the chromosomal body in the course of numerous cell divisions. Owing to printomeres, cellular memory about the proper structure of chromatin decompactions is created, kept, and can be carried through the succession of doublings of differentiated cells.
KEY WORDS: cellular memory, differentiation, fountain RNA, printomere