Rapid activation of ATM on DNA flanking double-strand breaks

The tumour-suppressor gene ATM, mutations in which cause the human genetic disease ataxia telangiectasia (A-T), encodes a key protein kinase that controls the cellular response to DNA double-strand breaks (DSBs). DNA DSBs caused by ionizing radiation or chemicals result in rapid ATM autophosphorylation, leading to checkpoint activation and phosphorylation of substrates that regulate cell-cycle progression, DNA repair, transcription and cell death. However, the precise mechanism by which damaged DNA induces ATM and checkpoint activation remains unclear. Here, we demonstrate that linear DNA fragments added to Xenopus egg extracts mimic DSBs in genomic DNA and provide a platform for ATM autophosphorylation and activation. ATM autophosphorylation and phosphorylation of its substrate NBS1 are dependent on DNA fragment length and the concentration of DNA ends. The minimal DNA length required for efficient ATM autophosphorylation is approximately 200 base pairs, with cooperative autophosphorylation induced by DNA fragments of at least 400 base pairs. Importantly, full ATM activation requires it to bind to DNA regions flanking DSB ends. These findings reveal a direct role for DNA flanking DSB ends in ATM activation

Molecular cloning and functional characterization of a cDNA encoding nucleosome assembly protein 1 (NAP-1) from soybean

NAP-1, a protein first isolated from mammalian cells, can introduce supercoils into relaxed circular DNA in the presence of purified core histones. Based on its in vitro activity, it has been suggested that NAP-1 may be involved in nucleosome assembly in vivo. We isolated a cDNA clone encoding a soybean NAP-I homolog, SNAP-1. The SNAP-1 cDNA contains an open reading frame of 358 amino acid residues with a calculated molecular weight of 41kDa. The deduced amino acid sequence of SNAP-1 shares sequence similarity with yeast NAP-1 (38%) and human hNRP (32%). Notable features of the deduced sequence are two extended acidic regions thought to be involved in histone binding. SNAP-1 expressed in Escherichia coli induces supercoiling in relaxed circular DNA, suggesting that SNAP-I may have nucleosome assembly activity. The specific activity of SNAP-1 is comparable to that of HeLa NAP-1 in an in vitro assay. Western analysis reveals that SNAP-I is expressed in the immature and young tissues that were examined, while mature tissues such as old leaves and roots, show very little or no expression. NAP-1 homologs also appear to be present in other plant species.close181