ENRICHMENT OF MIDDLE REPETITIVE ELEMENT BM-1 TRANSCRIPTS IN TRANSLATIONALLY ACTIVE RNA FRACTIONS OF THE SILKMOTH, BOMBYX-MORI

The Bm-1 repetitive element family represents a group of transcribed r epetitive sequences in the genome of the silkmoth Bombyx mori. In the Bm-5 and BmN permanent cell lines studied here, a-amanitin inhibition and nuclear 'run-on' experiments demonstrated that approximately 80% o f the Bm-1 transcripts are produced by RNA polymerase II. Bm-1 transcr ipts are dramatically enriched in poly A(+) and polysomal RNA fraction s compared to total RNA in these two cell lines. In the Bm-5 cell line , from total to poly A(+) and polysomal RNA fractions, Bm-1 transcript s are enriched approximately 4 and 2 times, respectively, while in the BmN cell line these same fractions are enriched about 2 and 19 times compared to total RNA. This suggests that the Bm-1 transcripts may be involved in post-transcriptional processes or control of translation. Our data also revealed less size heterogeneity of Bm-1 transcripts in polysomal as compared to nuclear fractions. In the Bm-5 and BmN cell l ines, the size of most transcripts containing Bm-1 sequences increases from approximately 1700 nt in the nucleus to 3000 nt in the polysomal fraction, both fractions with RNA much larger than the Bm-1 consensus sequence (250 bp). This raises the possibility that some Bm-1 element s are transcribed as part of larger transcripts containing mRNA by way of 'read-through', and may be involved in post-transcriptional regula tion of gene expression as cis and/or trans acting elements.