Histones in perspective

Histones occur in equal amounts to DNA in the cell nucleus and are largely responsible for the compaction of the genome into chromatin via the formation of nucleosomes and higher-order structures. Whereas two of the five histone types exhibit little structural variation, the remaining three occur in many variant tissue- or species-specific forms. Multiple postsynthetic enzymatic modifications accompanying virtually any type of genome activity, together with the programmed appearance of many histone variants during sea urchin embryogenesis (and other differentiation events in a number of organisms) make histones a challenging enigma in eukaryotic molecular biology.     

Histones in crenarchaea

Archaeal histone-encoding genes have been identified in marine Crenarchaea. The protein encoded by a representative of these genes, synthesized in vitro and expressed in Escherichia coli, binds DNA and forms complexes with properties typical of an archaeal histone. The discovery of histones in Crenarchaea supports the argument that histones evolved before the divergence of Archaea and Eukarya.     

Histones in protistan evolution

The potential of comparative studies on histones for use in protistan evolution is discussed, using algal histones as specific examples. A basic premise for the importance of histones in protistan evolution is the observation that these proteins are completely absent in prokaryotes (and cytoplasmic organelles), but with few exceptions, the same five major histone types are found in all higher plants and animals. Since the histone content of the algae and other protists is not constant, some of these organisms may represent transition forms between the prokaryotic and eukaryotic modes of packaging the genetic material. Comparative studies of protistan histones may thus be of help in determining evolutionary relationships. However, several problems are encounter with protistan histones, including difficulties in isolating nuclei, proteolytic degradation, anomalous gel migration of histones, and difficulties in histone identification. Because of the above problems, and the observed variability in protistan histones, it is suggested that several criteria be employed for histone identification in protists.     

Histones in cytological preparations

The presence of histones in fixed cytological preparations has been examined in order to investigate whether histones could be involved in G-banding. Up to 80% of the total cellular histones are removed by fixation. The remaining 20%, resistant to extraction by fixation, consist principly of histone fractions f1 and f2b. Dilute acid fails to remove quantitatively the histones that remain in cytological preparations after fixation. Trypsin treatment that results in banded patterns alters the presence of the remaining histones but heat treatment that results in banding fails to extract the remaining histones.     

Direct Effect of Sodium Nitrite on Extracted Histones

Irpex lacteus milk-clotting enzyme hydrolyzed the Phe(105)-Met(106) bond of κ-casein, causing the precipitation of para-κ-casein along with other casein fractions in the presence of calcium ions, with a mechanism similar to other milk-clotting enzymes. Furhtermore, Irpex enzyme hydrolyzed at the positions Leu(79)-Ser(80) and Tyr(30)-Val(31) of para-κ-casein.

Degradation patterns of β-casein by Irpex and Mucor miehei enzymes were almost the same by polyacrylamide gel electrophoresis, but Endothia parasitica enzyme showed a different degradation pattern. Under the conditions employed, β-casein appeared to be scarcely hydrolyzed by chymosin.

Comparing the specificity of Irpex enzyme on β-casein with that of chymosin, the common cleaving points were Leu(165)-Ser(166), Ala(189)-Phe(190), and Tyr(192)-Glu(193). The difference in the specificity between the enzymes was exhibited in the cleavage at the Leu(139)-Leu(140) bond by chymosin and of the Ser(142)-Trp(143) bond by Irpex enzyme. Although the cleaving points of β-casein by both enzymes resembled each other, each enzyme exhibited different degradation patterns of β-casein because of thier different order of cleavage.     

Histones with high lysine content

1. The preparation and properties of lysine-rich histones, which differ in a number of respects from the classical arginine-rich histones, have been described. 2. Lysine-rich histones, like those previously known, are located in cell nuclei. 3. Lysine-rich histones dissociate more readily from combination with nucleic acid than do other histones.