Complete sequence of the 10.3 kb silkworm Attacus ricini rDNA repeat, determination of the transcriptional initiation site and functional analysis of the intergenic spacer.

A complete sequence of the 10.3 kb silkworm rDNA repeat in Attacus ricini (also called as Samia cynthia ricini) was first determined here, and sequences coding for 18S, 5.8S and 28S were also mapped to rDNA unit. The precise transcription initiation site was determined by primer extension experiment. Intergenic spacer (IGS) in the length of 2670 bp in which transcription of 45s rRNA precursor starts was found to contain three distinct AT-rich (A + T > 75%) regions. The AT-richest region (from -320 to -3, A + T > 83%) is just in front of the transcription initiation site. Many short sequence homologies were also found to exist in the transcription initiation region. It clearly demonstrates that the nucleotide sequence of initiation region of rRNA shows great diversity among different species, but the general functional architectures show some conservation. Analyzing the IGS region of A. ricini may provide new insights into the mechanisms of the expression regulation of RNA polymerase I contributed genes.