engleski

Transcription of highly abundant heterochromatic sequences in some insect species

PRAT, PSUB and PGEN are highly abundant mutally unrelated sequences 142 bp, 144 bp and 429 bp long, respectively, which comprise from 20% to 40% of the genomes of coleopteran insect species Palorus ratzeburgii, P. subdepressus and P. genalis. In the form of long tandem repeats they encompass pericentromeric heterochromatin as well as centromeric regions of all chromosomes. These sequences are also widely spread among taxonomicaly distant coleopteran species as low copy number pericentromeric repeats and show extreme interspecific sequence conservation despite the fact that some of the species are separated for an evolutionary period of up to 60 Myr. Long evolutionary preservation and high sequence conservation suggest a possible functional significance. Transcription of PRAT, PSUB and PGEN was examined in all three developmental stages (larvae, pupae and adults) using RT-PCR and Northern hybridization. Multiple transcripts deriving from both DNA strands having approximate size between 5 kb and 1 kb are observed. The transcripts from both DNA strands have identical size profile and abundance suggesting that they are mostly in the form od double stranded RNA. The heterogenous size distribution of transcripts could be due to their rapid degradation or because of multiple start and stop sites of transcription. The transcripts are detected in all developmental stages but are the most abundant in adults, indicating their developmental regulation. Using primer extension analysis we have tried to map major initiation sites for transcripts deriving from each strand of PRAT, PSUB and PGEN. There seems to be a single transcription initiation site present within each of these sequences. For both DNA strands of an element transcription initiation sites are close to each other, indicating that the bidirectional transcription could be under the control of the same promoter. Although by Northern hybridization we were not able to detect small RNAs homologous to PRAT, PSUB and PGEN elements, we propose that long transcripts are degraded into siRNAs which possibly could affect heterochromatin formation.

transcription; siRNA; satellite DNA

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