The effect of cell stresses upon the expression of the Bm1 short interspersed element (SINE) family in cultured silk worm cells is examined. Primer extension analysis shows that Bm1 repeats are transcribed by RNA polymerase III (Pol III) into cytoplasmic RNAs. Five consecutive T residues, which would normally terminate Pol III transcription, occur within the Bm1 consensus and are included within cDNA sequences representing these transcripts. In analogy to mammalian SINEs, the level of the Bm1 transcripts increases in response to either heat shock, inhibiting protein synthesis by cycloheximide or viral infection. The lifetime of Bm1 RNA increases following cell insults so that post-transcriptional events partially account for stress induced increases in its abundance. In the case of heat shock, the increase in Bm1 RNA follows the transient increase in hsp70 mRNA indicating that this response is temporally regulated to occur later in heat shock recovery. These results support the proposal that SINE RNAs serve a role in the cell stress response that predates the divergence of insects and mammals implying that SINEs are essentially a class of cell stress genes.
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