Incorporation of chromosomal proteins HMG-14/HMG-17 into nascent nucleosomes induces an extended chromatin conformation and enhances the utilization of active transcription complexes.

Abstract

The role of chromosomal proteins HMG‐14 and HMG‐17 in the generation of transcriptionally active chromatin was studied in a Xenopus laevis egg extract which supports complementary DNA strand synthesis and chromatin assembly. Chromosomal proteins HMG‐14/HMG‐17 enhanced transcription from a chromatin template carrying a 5S rRNA gene, but not from a DNA template. The transcriptional potential of chromatin was enhanced only when these proteins were incorporated into the template during, but not after, chromatin assembly. HMG‐14 and HMG‐17 stimulate transcription by increasing the activity, and not the number, of transcribed templates. They unfold the chromatin template without affecting the nucleosomal repeat or decreasing the content of histone B4. We suggest that HMG‐14/HMG‐17 enhance transcription by inducing an extended conformation in the chromatin fiber, perhaps due to interactions with histone tails in nucleosomes. By disrupting the higher order chromatin structure HMG‐14/HMG‐17 increase the accessibility of target sequences to components of the transcriptional apparatus.

HMG17 is a chromatin-specific transcriptional coactivator that increases the efficiency of transcription initiation.

“…[The increase in the repeat length in the presence of HMG17 might be related to the finding that phosphorylated HMG14/17 can, under certain conditions, increase the repeat length of reconstituted chromatin from 145 to---160--165 bp (Drew 1993;Tremethick and Drew 1993}, but it is difficult to compare the data because of the significant differences in the chromatin reconstitution systems used in the other work. Note also that our observation of the increase in the repeat length in the presence of HMG 17 is in contrast to the interpretation of the micrococcal nuclease digestion data of previous studies, wherein it was concluded that incorporation of HMG14/17 into chromatin does not affect the repeat length (Crippa et al 1993;Trieschmann et al 1995). We suggest, however, that the differences may be in the interpretation of the data because in the earlier work, the micrococcal nuclease digestion ladders contained only three or four bands, which led to difficulty in the determination of the nucleosomal repeat lengths.]…”

“…Hence, the structural analysis revealed that HMG 17 was efficiently and properly incorporated into chromatin that was assembled with double-stranded, circular plasmid DNA, and these data thus indicate that the HMG17-mediated transcriptional activation was not attributable to a general inhibition of chromatin assembly. In other work, a Xenopus egg extract was used to prepare HMG17-containing chromatin for the analysis of the effect of HMG 17 on transcription by RNA polymerase III (Crippa et al 1993;Trieschmann et al 1995). In those experiments, chromatin was generated from a single-stranded-circular DNA template by a coupled DNA replication (complementary strand synthesis)-chromatin assembly process that was carried out in the absence of Mg(II)-ATP.…”

“…Biochemical experiments with SV40 minichromosomes had led to the conclusion that HMG14 stimulates transcriptional elongation but not initiation (Ding et al 1994). In addition, studies of HMG17-containing chromatin templates had indicated that HMG17 stimulates transcription by RNA polymerase III (Crippa et al 1993;Trieschmann et al 1995). Thus, the available data suggest a role for HMG14/17 in transcriptional regulation.…”

Nucleus size in the host cells of an Arbuscular Mycorrhizal system: a mathematical approach to estimate the role of ploidy and chromatin condensation

“…Chromatin condensation is controlled by several factors, including reversible acetylation of histone H3 and H4 (Allfrey et al 1964;Turner 1991;Lee et al 1993;Loidl 1994;Braunstein 1996;Roth and Allis 1996;Grunstein 1997;Wade et al 1996;Mizzen and Allis 1996), methylation of DNA cytosine in position 5 (Cacciamani et al 2002), interactions with HMG (Crippa et al 1993;Paranjape et al 1995;Trischmann et al 1995;Ding et al 1997;Klosterman and Hadwiger 2002) or polycomb proteins (Preuss 1999) and with histone H1 (Wolffe 1997;Dou and Gorovsky 2000) The last one is involved in the first level of chromatin condensation, therefore its role seems especially relevant.…”

The In Vitro Reconstitution of Nucleosome and its Binding Patterns with HMG1/2 and HMG14/17 Proteins

“…It was known that HMG 14/17 proteins could enhance the replication and transcription when the chromatin was used as template [20].…”