Minichromosome

Koshland D, Hartwell LH 1987 The structure of sister minichromosome DNA before anaphase in Saccharomjces cerevisiae. Science 238 1713—1716 KumadaK, Nakamura T, Nagao K, Funabiki H, Nakagawa T, Yanagida M 1998 Cutl is loaded onto the spindle by binding to Cut2 and promotes anaphase spindle movement upon Cut2 proteolysis. Curr Biol 8 633—641... 

Shroyer KR, Homer P, Heinz D, et al. Validation of a novel immunocytochemical assay for topoisomerase II- and minichromosome maintenance protein 2 expression in cervical cytology. Cancer 2006 108 324-330. 

However, the packing of DNA into nucleosome-like structures is not unique to eukarya similar structures appear in archaea (reviewed in Reeve et al., 1997). Additionally, histones and minichromosome maintenance proteins (MCM) are widespread among eukarya and archaea and absent in prokarya, and the eukaryotic chromo domain has a structure that is highly reminiscent of archaeal histones that are involved in formation of archaeal chromatin (Ball et al., 1997). Consequently, it is possible that chromatin remodeling in eukaryotes is an elaboration of a similar cellular mechanism in archaea. 

The recognition of positioning sequences has also made possible the construction of minichromosomes of regular defined structure (e.g., Ref. [44]), and reconstituted nucleosomes containing a defined DNA sequence. This latter advance was essential for the high-resolution X-ray diffraction studies of nucleosomes that have been accomplished (see Section 2.1). Defined minichromosomes have proved a powerful tool in many studies for a recent example, see Fan et al. [45]. 

This problem first emerged from the necessity to reconcile topological and structural data of nucleosomes and chromatin. As soon as a minichromosome could be reconstituted from pure DNA and histones, the total reduction of the DNA linking number (Lk) was found to be equal to the number of nucleosomes, which was also true for the native Hl-bearing SV40 minichromosome [12]. On the other hand, the first low-resolution crystal of the core particle showed that DNA was wrapped with 1 3/4 turns of a left-handed superhelix. Assuming linker DNAs... 

The advantage of such a simple system was twice first, to suppress a potential linker contribution to the mean ALk per nucleosome, termed (ALkn) from now on, which could result from nucleosome/nucleosome interactions in a minichromosome ... 

Morse, R.H. and Cantor, C.R. (1986) Effect of trypsinization and histone H5 addition on DNA twist and topology in reconstituted minichromosomes. Nucl. Acids Res. 14, 3293-3310. 

The C-terminal domain of HMGN was required for transcriptional enhancement in both SV40 minichromosomes and in minichromosomes assembled using... 

A second model for HMGN action is that it counteracts chromatin compaction by linker histones. The ability of HMGN proteins to unfold SV40 minichromosomes and stimulate transcription from them is dependent on the presence of linker histones, and the data is consistent with HMGN counteracting the repressive... 

HMGN proteins are small, ubiquitous, chromatin architectural elements that bind to the 147 bp nucleosome core via their highly conserved nucleosome binding domain. They bind as homodimers, interacting near base 25 of the nucleosome DNA, and near the dyad axis. When incorporated into minichromosomes, HMGN proteins decrease chromatin folding in a manner that is dependent on their C-terminal domain, with a concomitant increase in transcription. 

Glotov, B.O., Rudin, A.V., and Severin, E.S. (1982) Conditions for sliding of nucleosomes along DNA SV40 minichromosomes. Biochim. Biophys. Acta 696, 275-284. 

As mentioned in Section B, 1, human centromeres are rich in the repetitive a-satellite DNA. By joining a-satellite DNA-containing fragments of the X-chro-mosome to cloned telomeric DNA, human minichromosomes have been created.488 These have been developed into human artificial chromosomes,489 which may be practical vehicles for gene transfer in human therapy. 

Another difference between bacterial and eukaryotic replication is the presence of nucleosomes in eukaryotes. Some evidence suggests that nucleosomes may open and close to allow replication forks to pass through.509 Studies of SV40 minichromosomes indicate that passage of the replication machinery does destabilize nucleosomes, which must be partially reconstructed about 260 nucleotides past the elongation point.510 Another factor is the variable extent and location of modifications to histones, in particular to... 

With circular minichromosomes containing this unique segment termed oriC, it has been possible to study the initiation and replication process in vitro. First, in crude extracts it was possible to show that replication initiates within or near the oriC sequence and proceeds bidirectionally. About 13 different proteins participate in the oriC-directed DNA replication as judged by their activity in a reconstituted replication reaction. 

Fig. 12.15. Effects of polyamines on the OH-induced strand-breakage of SV40 DNA (putrescine open triangles, spermine filled diamonds) and SV40 minichromosomes (spermine open squares) according to Newton et al. (1996, with permission)...

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