Evidence for bacterial origin of heat shock RNA-1.

The heat shock RNA-1 (HSR1) is a noncoding RNA (ncRNA) reported to be involved in mammalian heat shock response. HSR1 was shown to significantly stimulate the heat-shock factor 1 (HSF1) trimerization and DNA binding. The hamster HSR1 sequence was reported to consist of 604 nucleotides (nt) plus a poly(A) tail and to have only a 4-nt difference with the human HSR1. In this study, we present highly convincing evidence for bacterial origin of the HSR1. No HSR1 sequence was found by exhaustive sequence similarity searches of the publicly available eukaryotic nucleotide sequence databases at the NCBI, including the expressed sequence tags, genome survey sequences, and high-throughput genomic sequences divisions of GenBank, as well as the Trace Archive database of whole genome shotgun sequences, and genome assemblies. Instead, a putative open reading frame (ORF) of HSR1 revealed strong similarity to the amino-terminal region of bacterial chloride channel proteins. Furthermore, the 5' flanking region of the putative HSR1 ORF showed similarity to the 5' upstream regions of the bacterial protein genes. We propose that the HSR1 was derived from a bacterial genome fragment either by horizontal gene transfer or by bacterial infection of the cells. The most probable source organism of the HSR1 is a species belonging to the order Burkholderiales.