Mammalian H3.3 is a variant of the canonical histone H3.1 essential for genome reprogramming in the fertilized eggs and maintenance of chromatin structure in neuronal cells. An H3.3-specific histone chaperone, DAXX, directs the deposition of H3.3 onto pericentric and telomeric heterochromatin. H3.3 differs from H3.1 by only five amino acids, yet DAXX can distinguish the two with high precision. By a combination of structural, biochemical and cell-based targeting analyses, here we show that Ala87 and Gly90 are the principal determinants of H3.3 specificity. DAXX uses a shallow hydrophobic pocket to accommodate the small, hydrophobic Ala87 of H3.3, whereas a polar binding environment in DAXX prefers Gly90 in H3.3 over the hydrophobic Met90 in H3.1. An H3.3-H4 heterodimer is bound by the histone-binding domain of DAXX, which makes extensive contacts with both H3.3 and H4.

The H3 histone variant CENP-A is an epigenetic marker critical for the centromere identity and function. However, the precise regulation of the spatiotemporal deposition and propagation of CENP-A at centromeres during the cell cycle is still poorly understood. Here, we show that CENP-A is phosphorylated at Ser68 during early mitosis by Cdk1. Our results demonstrate that phosphorylation of Ser68 eliminates the binding of CENP-A to the assembly factor HJURP, thus preventing the premature loading of CENP-A to the centromere prior to mitotic exit. Because Cdk1 activity is at its minimum at the mitotic exit, the ratio of Cdk1/PP1α activity changes in favor of Ser68 dephosphorylation, thus making CENP-A available for centromeric deposition by HJURP. Thus, we reveal that dynamic phosphorylation of CENP-A Ser68 orchestrates the spatiotemporal assembly of newly synthesized CENP-A at active centromeres during the cell cycle.

Specific recognition of centromere-specific histone variant CENP-A-containing chromatin by CENP-N is an essential process in the assembly of the kinetochore complex at centromeres prior to mammalian cell division. However, the mechanisms of CENP-N recruitment to centromeres/kinetochores remain unknown. Here, we show that a CENP-A-specific RG loop (Arg80/Gly81) plays an essential and dual regulatory role in this process. The RG loop assists the formation of a compact "ladder-like" structure of CENP-A chromatin, concealing the loop and thus impairing its role in recruiting CENP-N. Upon G1/S-phase transition, however, centromeric chromatin switches from the compact to an open state, enabling the now exposed RG loop to recruit CENP-N prior to cell division. Our results provide the first insights into the mechanisms by which the recruitment of CENP-N is regulated by the structural transitions between compaction and relaxation of centromeric chromatin during the cell cycle.

The centromere regulates proper chromosome segregation and its dysfunction is implicated in chromosomal instability (CIN). However, relatively little is known about how centromere dysfunction occurs in cancer. Here we define the consequences of phosphorylation by Cyclin E1/CDK2 on a conserved Ser18 residue of centromere-associated protein CENP-A, an essential histone H3 variant that specifies centromere identity. Ser18 hyperphosphorylation in cells occurred upon loss of FBW7, a tumor suppressor whose inactivation leads to chromosomal instability (CIN). This event on CENP-A reduced its centromeric localization, increased CIN and promoted anchorage-independent growth and xenograft tumor formation. Overall, our results revealed a pathway that Cyclin E1/CDK2 activation coupled with FBW7 loss promotes CIN and tumor progression via CENP-A-mediated centromere dysfunction.

During meiotic prophase, chromosomes pair and synapse with their homologs and undergo programmed DNA double-strand break (DSB) formation to initiate meiotic recombination. These DSBs are processed to generate a limited number of crossover recombination products on each chromosome, which are essential to ensure faithful segregation of homologous chromosomes. The nematode Caenorhabditis elegans has served as an excellent model organism to investigate the mechanisms that drive and coordinate these chromosome dynamics during meiosis. Here we focus on our current understanding of the regulation of DSB induction in C. elegans. We also review evidence that feedback regulation of crossover formation prolongs the early stages of meiotic prophase, and discuss evidence that this can alter the recombination pattern, most likely by shifting the genome-wide distribution of DSBs.

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A novel phenylacetic acid (PAA)-induced CoA-ligase-encoding gene, designated as phlC, has been cloned from penicillin-producing fungus Penicillium chrysogenum. The open reading frame of phlC cDNA was 1671 bp and encoded a 556 amino acid residues protein with the consensus AMP binding site and a peroxisomal targeting signal 1 on its C terminus. The deduced amino acid sequence showed 37% and 38% identity with characterized P. chrysogenum Phl and PhlB protein, respectively. Functional recombinant PhlC protein was overexpressed in Escherichia coli. The purified recombinant enzyme was capable to convert PAA into its corresponding CoA ester with a specific activity of 129.5 ± 3.026 pmol/min per mg protein. Similar to Phl and PhlB, PhlC displayed broad substrate spectrum and showed higher activities to medium- and long-chain fatty acids. The catalytic properties of PhlC have been determined and compared to those of Phl and PhlB.

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