CHICKEN MICROCHROMOSOMES ARE HYPERACETYLATED, EARLY REPLICATING, AND GENE RICH

The chicken karyotype consists of 39 chromosomes of which 33 are class ed as microchromosomes [MICs]. MICs contain about one third of genomic DNA. The majority of mapped chicken genes are assigned to macrochromo somes (MACs), but a recent study indicated that CpG islands (CGIs), wh ich are associated with most vertebrate genes, map predominantly to MI Cs. The present work establishes that chicken genes are concentrated o n MICs by several criteria. Acetylated (lysine 5) histone H4, which is strongly correlated with the presence of genes, is highly enriched on MICs by immunocytochemistry. In addition, detailed analysis of chicke n cosmids shows that CGI-like fragments are approximately six times de nser on MICs than on MACs. Published mapping of randomly chosen genes by fluorescent in situ hybridization (FISH) also shows a significant e xcess of microchromosomal assignments. Finally, the finding that MICs replicate during the first half of S phase is also compatible with the suggestion that MICs represent gene-rich DNA. We use the cosmid data to predict that similar to 75% of chicken genes are located on microch romosomes.