Transcription of pericentromeric heterochromatin in beetles--satellite DNAs as active regulatory elements

Tenebrionid beetles from the genus Palorus (Coleoptera) have a significant amount of heterochromatin in pericentromeric regions of all chromosomes. The major DNA component of pericentromeric heterochromatin is a highly abundant satellite DNA. Analysis of transcription of a major satellite DNA PSUB from species Palorus subdepressus reveals a constitutive level of expression similar in all 3 developmental stages: larvae, pupae and adults, corresponding to 0.01% of total RNA. Transcription proceeds from both DNA strands in equal amounts resulting in long heterogeneous size transcripts ranging in size from 500 bp to more than 5 kb. Although equal transcription from both DNA strands could potentially activate the RNA interference (RNAi) pathway processing of long PSUB transcripts into small interfering RNAs (siRNA) was not detected. Transcripts preferentially remain in the nucleus and 90% of them are not polyadenylated. Transcription initiation sites, mapped by primer extension, are located within PSUB satellite monomers as well as motifs characteristic for RNA polymerases II and III. Putative RNA polymerase II promoter, predicted by computational approach, shares a 65% sequence similarity to the Pol II promoter mapped previously in PRAT satellite DNA, a major satellite of related species P. ratzeburgii. Results give strong indications that Palorus satellite DNAs are transcribed as autonomous transcription units from their own promoters that reside within satellite sequences. Long satellite DNA transcripts remain mostly in the nucleus and are proposed to play a structural role in the organization of pericentromeric heterochromatin.