| Summary: | In <i>Drosophila</i> salivary gland polytene chromosomes, a substantial portion of heterochromatin is underreplicated. The combination of mutations <i>SuUR<sup>ES</sup></i> and <i>Su(var)3-9<sup>06</sup></i> results in the polytenization of a substantial fraction of unique and moderately repeated sequences but has almost no effect on satellite DNA replication. The Rap1 interacting factor 1 (Rif) protein is a conserved regulator of replication timing, and in <i>Drosophila</i>, it affects underreplication in polytene chromosomes. We compared the morphology of pericentromeric regions and labeling patterns of in situ hybridization of heterochromatin-specific DNA probes between wild-type salivary gland polytene chromosomes and the chromosomes of <i>Rif1</i> mutants and <i>SuUR Su(var)3-9<sup>06</sup></i> double mutants. We show that, despite general similarities, heterochromatin zones exist that are polytenized only in the <i>Rif1</i> mutants, and that there are zones that are under specific control of <i>Su(var)3-9</i>. In the <i>Rif1</i> mutants, we found additional polytenization of the largest blocks of satellite DNA (in particular, satellite 1.688 of chromosome X and simple satellites in chromosomes X and 4) as well as partial polytenization of chromosome Y. Data on pulsed incorporation of 5-ethynyl-2′-deoxyuridine (EdU) into polytene chromosomes indicated that in the <i>Rif1</i> mutants, just as in the wild type, most of the heterochromatin becomes replicated during the late S phase. Nevertheless, a significantly increased number of heterochromatin replicons was noted. These results suggest that <i>Rif1</i> regulates the activation probability of heterochromatic origins in the satellite DNA region.
|
|---|