is and this modification is catalyzed by the meiotic Mek1 kinase.120,121 In contrast to the H3S10A substitution in yeast that strikingly had no negative effect on sporulation, the T11A mutant showed a reduction in sporulation efficiency, arguing that phosphorylation of T11 is a key modification during meiosis.121 Phosphorylation of H4S1 and H2BS10 was also demonstrated to occur during meiosis in yeast. Additionally, H4S1ph has been observed during fruit fly and mouse spermatogenesis.80 Examination of H4S1ph 
timing relative to H3S10 phosphorylation shows that these two marks have a distinct pattern during sporulation.80 Unlike the transient H3S10ph that coincides with early sporulation, H4S1ph appears as a stable mark after meiotic I and II divisions.80,121 In fact, S10 residues of both H2B and H3 are phosphorylated during meiotic chromosome condensation and disappear during meiotic divisions,122 whereas H4S1 phosphorylation appears later in meiosis and increases in post-meiotic cells.80,81 H4S1ph is stable and persists in post-meiotic cells, including mature yeast spores and mature spermatids. This meiotic phosphorylation is dependent on Sps1, a mid-sporulationspecific kinase member of the Ste20 family.80 There is no in vitro evidence that Sps1 phosphorylates H4S1 directly, but deletion of this kinase results in a loss of H4S1ph signal. Supporting the role of H4S1ph in chromatin compaction during later stages of meiosis, deletion of Sps1 as well as mutation of H4S1 to alanine in yeast results in sporulation deficiency and increased DNA and nuclear volumes.80,121 Finally, H2BS10 phosphorylation has been detected during the prophase of meiosis, although the kinase responsible for this modification in meiosis remains unknown.122 While Ipl1 can phosphorylate both H3 and H2B in vitro, the possibility that PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19811292 it phosphorylates H2B on S10 in vivo during meiosis remains to be addressed. Intriguingly, this histone mark was found to colocalize with Zip1, a component of the synaptonemal complex characteristic of meiotic pachytene chromosomes. Overall, current evidence indicates that H2BS10ph plays a major role in chromatin condensation during various cellular processes such as meiosis but the molecular mechanism by which this modification influences chromatin structure still needs to be Pyrroloquinolinequinone disodium salt biological activity elucidated. Phosphorylation of linker histone H1 is also regulated in a cell cycle-dependent manner. It significantly increases during mitosis and S phase, being more specifically detected through metaphase. Multiple phosphorylated sites have been identified84,124 and the state of H1 phosphorylation is thought to be dependent on the balance of protein phosphatase I and CDC2/CDK2 kinases.125 H1 phosphorylation appears to be associated with chromatin decondensation rather that chromatin condensation.123 While H1 was shown to bind HP1 and to stabilize the compacted chromatin structure, its phosphorylation by CDK2 is proposed to disrupt this binding, resulting in chromatin destabilization and efficient cell-cycle progression.126 Whereas N- and C-terminal tails of H1 contain consensus sequences with potential phosphorylatable sites by CDC2/ CDK2 kinases, it is suggested that it is the number of phosphorylated sites that is functionally important, not the specific residues per se.127 Histone phosphorylation and chromatin compaction associated with apoptosis. Several intriguing connections exist between histone phosphorylation and apoptosis. Early work demonstrated that phos