High mobility group proteins 14 and 17 can space nucleosomes in vitro

Recently we partially purified from Xenopus laevis ovaries a novel, ATP-dependent, spacing activity that can convert a DNA template consisting of irregularly spaced nucleosomes into a chromatin structure made up of regularly spaced nucleosomes with a repeat length of 160-165 base pairs. In a second independent step, the longer spacing of higher eukaryotic chromatin can be generated by the addition of histone H1. The partially purified spacing fraction contains several proteins that display chromatographic properties and mobilities on polyacrylamide gels similar to high mobility group (HMG) proteins. For that reason, different HMG proteins were tested for their ability to generate chromatin structures with regularly spaced nucleosomes. In this report, using two different nucleosome assembly systems, we show that the addition of phosphorylated HMGs 14 and 17 to the histone octamer results in the formation of chromatin with a repeat length of 160-165 base pairs. The results are similar to those obtained from studies of chromatin structure in simple cells, such as fungi and yeast, and in active genes.