Enhancement of the Transcription Potential of Nascent Chromatin by Chromosomal Proteins HMG-14/-17 Is Coupled to Nucleosome Assembly and Not DNA Synthesis

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Abstract
We have previously demonstrated that in Xenopus egg extracts, which support DNA strand synthesis and chromatin assembly, incorporation of chromosomal proteins HMG-14/-17 into nascent nucleosomes increases the transcriptional potential of a chromatin template carrying the Xenopus 5S RNA gene. Here we use the single-stranded and double-stranded forms of a plasmid carrying a 5S RNA maxigene, to test whether the effect of HMG-14/-17 on transcription requires DNA synthesis and whether these proteins will affect transcription through a region containing nucleosomes. We find that most of the transcripts were about 350 nucleotides long, suggesting that HMG-14/-17 enhance transcription through a region that could contain nucleosomes. HMG-14/-17 enhance transcription of chromatin templates assembled onto double-stranded DNA, in the absence of DNA synthesis. Single-round transcription assays suggest that HMG-14/-17 increase transcription from templates assembled onto both single- and double-stranded DNA by increasing the specific activity, and not the number, of transcriptionally active templates. We conclude that the effect of HMG-14/17 on the transcriptional potential of chromatin is linked to nucleosome assembly and is not linked to DNA synthesis.