Dual modes of transcriptional and translational initiation of SSP1, the gene for a mitochondrial HSP70, responding to heat-shock in Schizosaccharomyces pombe.

The SSP1 gene of Schizosaccharomyces pombe which is homologous to the SSC1 gene of Saccharomyces cerevisiae was cloned and its nucleotide sequence determined. A heat-shock element and three possible TATA boxes were found upstream of the coding region. Dual modes of transcriptional initiation were observed in primer extension analyses using as templates the mRNAs prepared from cells before and after heat-shock. Initiation sites situated 50 to 60 nucleotides downstream of the normal one were found to be additionally used in cells after heat-shock. Thus, the mode of transcription in heat-shocked cells of S. pombe appears to be more similar to the one observed in higher eukaryotes than that in S. cerevisiae. The SSP1 gene contains two methionine codons in the region predicted to encode a mitochondrial targeting signal sequence of its protein (Ssp1) and the stretch between the two methionine codons is capable of forming stem-loop structures. Both of the two methionine codons were confirmed to function as translational initiators in vitro. A possible mechanism is proposed for the response of the SSP1 gene towards heat-shock by the differential initiation of its transcription and translation.