In C. elegans, proper chromosome alignment and synapsis initiation during meiosis are mediated by special regions called "Pairing Centers" that lie near one end of each of the six chromosomes. We have shown that these regions have two separable activities - they stabilize homolog pairing and contribute to initiating synapsis. We previously characterized the protein HIM-8 as an essential trans-acting factor of the X chromosome Pairing Center. It is required for X chromosome pairing and synapsis and localizes specifically to the X chromosome Pairing Center.
him-8 mutations had no discernable effect on the segregation behavior of any of the autosomes.
him-8 is located in an unusual operon containing three additional genes with high homology to
him-8 and each other, which we have named
zim-1, -2, and -3 (for zinc finger in meiosis). More recently, we have demonstrated analogous roles for ZIM-1, -2, and -3 at the autosomal Pairing Centers through mutational analysis and immunolocalization. In addition, we have begun to examine the orthologs of ZIM/HIM-8 family in related nematodes. C. remanei, like C. elegans, has four members of this protein family, but C. briggsae, has five, indicating expansion/contraction of this cluster.
Recent efforts in our lab have focused on identifying the DNA elements that confer Pairing Center activity. We first showed that HIM-8 protein is recruited efficiently to an extrachromosomal array containing a 30-kb segment of the X chromosome, and have recently whittled this initial HIM-8 recruiting segment down to a 250 bp DNA fragment. Within this short segment are several copies of a 21-bp repeat that is highly enriched over an extensive region on the left end of the X chromosome, relative to the rest of the X chromosome or the autosomes. Based on this candidate sequence, we have examined the genomic regions corresponding to the autosomal Pairing Center regions for similarly enriched sequences. We are currently testing the affinity of these proteins for candidate sequences in vitro, using a fluorescent oligonucleotide binding assay.
This group of genes encodes the first chromosome-specific proteins implicated in homolog pairing and synapsis. Analysis of this family of proteins and their chromosomal binding sites is revealing new insights into the mechanisms that bring homologs together to ensure proper segregation.