Characterization of cis-elements in the promoter of trz2 encoding Schizosaccharomyces pombe mitochondrial tRNA 3′-end processing enzyme

The endonuclease tRNase Z is responsible for the 3'-end processing of tRNA precursors, which is one of the essential steps in tRNA maturation. The fission yeas Schizosaccharomyces pombe contains two essential tRNase Z L genes ( rz1 and rz2 ) involved in nuclear and mitochondrial tRNA 3'-end processing, respectively. Our previous studies suggest tha rz2 is expressed at a very low level. Here we report characterization of the rz2 promoter. Using lacZ as a reporter, we show that the rz2 promoter contains a HomolD box and a very weak diverged TATA element. The HomolD box is usually found in the promoters of S. pombe ribosomal protein genes. lacZ reporter assays suggest that the HomolD box regulates the expression of both rz2 and the ribosomal protein gene rps2501 , which are arranged head-to-head on opposite strands. Overexpression of Rrn7, a candidate HomolD box-binding protein, up-regulates expression of lacZ under the control of the rz2 promoter or the rps2501 promoter. Functional complementation studies suggest that the TATA-like element is essential for rz2 expression, whereas the HomolD box may play a nonessential regulatory role. We also demonstrate that a 57 nt negative regulatory element (NRE) located between the HomolD box and the TATA-like element represses the expression of lacZ under the control of the rz2 promoter. Our results suggest that the low-level rz2 expression may arise from a low level of transcription caused by lack of a strong TATA box and the NRE. Our analysis also suggests tha rz2 and rps2501 may be coregulated by the HomolD box.