Chromatin subunits ("nucleosomes") which were purified by sucrose gradient centrifugation of a staphylococcal nuclease digest of chromatin have been studied. We found that such a preparation contains nucleosomes of two discrete types which can be separated from each other by polyacrylamide gel electrophoresis. Nucleosome of the first type contains all five histones and a DNA segment of approximately 200 base pairs long, whereas nucleosome of the second type lacks histone H1 and its DNA segment is approximately 170 base pairs long, i.e., about 30 base pairs shorter than the DNA segment of the nucleosome of the first type. Purified dimer of the nucleosome also can be fractionated by gel electrophoresis into three discrete bands which correspond to dinucleosomes containing two molecules of histone H1, one and no H1. These and related findings strongly suggest that the H1 molecule is bound to a short (approximately 30 base pairs) terminal stretch of the nucleosomal DNA segment which can be removed by nuclease (possibly in the form of H1-DNA complex) without any significant disturbance of main structural features of the nucleosome.

Analysis of E.col chromosomes isolated under conditions similar to those used for isolation of eukaryotic chromatin has shown that: 1) The proteins of highly purified E.coli de-oxibonucleoprotein are mainly in addition to RNA polymerase two specific histone-like proteins of apparent molecular weight of 17 000 and 9,000 (proteins I and 2, respectively).2) Proteins 4 and 2 occur in approximately equal molar amounts in the isolated E coli chromosome, and their relative content corresponds to one molecule of protein I plus one molecule of protein 2 per 150-200 base pairs of DNA. 3) There are no long stretches of naked DNA in the purified E.coli deozyribonucleoprotein suggesting a fairly uniform distriution of the proteins I and 2 along DNA. 4) The protein 2 is apparently identical to the DNA-binding protein UJ which was isolated previously /1/ from extracts of E.coli cells. 5) Digestion of the isolated E.coli chromosomes itE staphylococcal nuclease proceeds throu-ghdiscrete deoxyribonucleoprotein intermediates (in particular, at~120 base pairs) which contain both proteins 1 and 2. However, since no repeating multimer structure was observed so far in nuclease digests of the E.coli chromosome, it seems premature to draw definite concl-usns about possible similarities between the nucleosomal organization of the eukaryotic chromatin and the E.coli chromatin structure.

In contrast to conclusions of previous studies /I-3/ claiming the absence of histone HI from the SV40 and polyoma viral minichromosomes we have found that a preparation of purified SV40 minichromosomes does contain histone HI. The content of HI in relation to other four histones in the SV40 minichromosomes is close to that in the cellular chromatin. Histone HI in the isolated SV40 minichromosomes is bound apparently to internucleosomal DNA stretches as was shown already for HI in the cellular chromatin /4/. In addition it was found that more than 90% of the purified SV40 minichromosomes migrated as a single discrete deoxyribonucleoprotein band upon agarose gel electrophoresis.

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We report two new findings bearing on the "supranucleo-somal" level of the structure of the Simian Virus 40 minichromosome. I) Isolated SV40 minichromosome which contains all five histones including HI/I/ exists in solution under approximately physiological ionic conditions as a compact roughly spherical particle approximately 300 A in diameter which is capable of fitting within the virus capsid. In spite of such a compact conformation of the minichromosome individual nucleosomes can be readily visualized within the particle. Compact state of SV40 minichromosome depends on both the presence of histone HI and maintenance of approximately physiological ionic strength of solution (micron approximately 0.15). Removal of HI results in a conversion of the compact minichromosomes into an extended (circular beaded) structure. 2) The compact form of the SV40 minichromosome in contract to its circular beaded form is virtually completely resistant to staphylococcal nuclease, strongly suggesting that in particular nuclease-sensitive parts of the internucleosomal DNA regions are not exposed on the outside of the compact SV40 minichromosome. On the other hand, DNase I which is known to attack both inter-and intranucleosomal DNA in the chronatin /2,3/ readily digests the compact form of the SV40 minichromosome. Possible models of the compact minichromosome and implications for higher order structures of the cellular chromatin are discussed.

The overall allele pattern and acetylator status distribution in Iranians displayed the considerable prevalence of "slow acetylators" over "rapid acetylators," similar to those of Caucasians except for a minor difference observed in the frequency of the NAT2*7 allele. Nucleic acid testing for common NAT2 mutations might be a potentially useful tool for an accurate phenotype interpretation and identification of Iranian individuals at risk.

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