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 yeast  Schizosaccharomyces pombe  contains two essential tRNase Z L  genes ( trz1  and  trz2 ) involved in nuclear and mitochondrial tRNA 3'-end processing, respectively. Our previous studies suggest that  trz2  is expressed at a very low level. Here we report characterization of the  trz2  promoter. Using  lacZ  as a reporter, we show that the  trz2  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  trz2  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  trz2  promoter or the  rps2501  promoter. Functional complementation studies suggest that the TATA-like element is essential for  trz2  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  trz2  promoter. Our results suggest that the low-level  trz2  expression may arise from a low level of transcription caused by lack of a strong TATA box and the NRE. Our analysis also suggests that  trz2  and  rps2501  may be coregulated by the HomolD box.
Liu J, Huang L, Wang Y, Huang Y