THE ORGANIZATION AND EXPRESSION DURING OOGENESIS OF HIGHLY REPETITIVE DNA IN THE NEWT NOTOPHTHALMUS VIRIDESCENS (SATELLITE DNA, IN SITU HYBRIDIZATION, LAMPBRUSH CHROMOSOMES)

A significant proportion of the genomes of higher eukaryotes is composed of DNA sequences which are reiterated many times. One type of repeated sequence, satellite DNA, consists of highly reiterated, tandemly repeated sequences which tend to be localized in large blocks in the transcriptionally inert heterochromatic regions of chromosomes.;By analyzing the ability of oocyte RNA to protect a cloned satellite DNA probe from S1 nuclease digestion, the transcription of both strands of satellite 1 has been confirmed. Transcripts containing satellite 1 are restricted to the nucleus of the oocyte. In situ hybridization to cytological sections of ovary demonstrates that satellite 1 transcripts are detectable by late pachytene. In addition, complementary to satellite 1 probes are present in the testis. Therefore, expression of this satellite is not exclusive to the oocyte, and may be a common feature in germ line cells.;Satellite 2 is an abundant 330 bp sequence which differs from satellite 1 both in its genomic organization and in its pattern of expression in the oocyte. In contrast to the predominantly heterochromatic localization of satellite 1, this sequence is dispersed throughout the genome in tandemly repeated arrays. Both strands of satellite 2 are also transcribed on lampbrush chromosome loops. Northern blot analysis has shown that satellite 2 probes hybridize to two small transcripts in ovary RNA.;Two highly reiterated satellite sequences which are expressed during oogenesis in the newt Notophthalmus viridescens have been cloned and characterized. Satellite 1, a 222 bp tandemly repeated sequence, is located in the pericentric heterochromatin of all chromosomes and at the sphere loci of chromosomes 2 and 6. It was previously shown by in situ hybridization to the nascent transcripts on lampbrush chromosome loops that both strands are transcribed at the sphere loci. Since this satellite flanks the histone gene clusters in the newt, it appears that transciption initiates on a histone gene promotor and proceeds without interruption into the adjacent repetitive sequence.