title: 3B55: a repetitious sequence family which is transcribed and proportionately replicated in germ-line polyploid nuclei of Calliphora erythrocephala.
abstract: The chromosomes of dipteran polyploid nurse cell nuclei are functionally analogous to the oocyte lampbrush chromosomes of the Amphibia. In investigating the transcriptional and replicative activity of these nuclei in Calliphora erythrocephala we have identified a cloned highly repetitious DNA fragment which shows enhanced transcriptional activity in these cells and different replicative behavior in these germ-line polyploid nuclei as opposed to somatic polytene nuclei. The clone, 3B55, contains a 6.8-kb insert which consists primarily of tandemly repeated 200-bp sequences defined by RsaI sites. From Southern hybridizations to diploid (embryonic) genomic DNA, 3B55-related DNA was calculated to represent a significant fraction of the haploid genome (0.8% or 5000 kb). In situ hybridizations established that these sequences are present in the pericentric regions of four of the six chromosomes. Thus the 3B55 sequences have the properties of a satellite-type DNA family. Quantitation of the 3B55 200-bp monomer (which represents approximately 60% of the genomic 3B55 DNA sequences in all tissues examined) revealed that in somatic polytene salivary gland nuclei, 3B55 DNA is highly under-replicated to give a monomer representation of only 48 +/- 11 kb per haploid genome. However, in germ-line nurse cell nuclei, 3B55 DNA is proportionately replicated to give a monomer genomic representation (3560 +/- 344 kb) equivalent to that of diploid DNA (3011 +/- 202 kb). Transcripts complementary to 3B55 sequences are at least 25 times more abundant in total nurse cell nuclear RNA than in total embryonic nuclear RNA. These findings suggest an association of the 3B55 sequence family with some germ-line specific function.
mesh_terms: D000818:Animals; D001482:Base Composition; D002467:Cell Nucleus / Q000032:analysis*; D002875:Chromosomes / Q000648:ultrastructure; D004247:DNA / Q000235:genetics*; D004261:DNA Replication; D004262:DNA Restriction Enzymes; D004175:Diptera / Q000196:embryology*; D004625:Embryo, Nonmammalian / Q000502:physiology; D009693:Nucleic Acid Hybridization; D011123:Polyploidy; D012091:Repetitive Sequences, Nucleic Acid; D014158:Transcription, Genetic
chemical_list: D004247:DNA; D004262:DNA Restriction Enzymes