We have previously reported the co-purification of a tRNA-like molecule with the Trypanosoma brucei SRP complex. To examine whether the trypanosome SRP has a unique composition compared with that of other eukaryotes, we analyzed the 7SL RNA and the SRP complex of the monogenetic trypanosomatid Leptomonas collosoma. The 7SL RNA from L.collosoma was cloned, and its gene was sequenced. In contrast to T:brucei, two 7SL RNA transcripts were detected in L.collosoma that originate from a single-copy gene. Using stable cell lines expressing tagged 7SL RNA, we demonstrate that the tRNA(Arg) gene located 98 bp upstream to the 7SL RNA serves as part of the 7SL RNA extragenic promoter. The steady-state level of 7SL RNA was found to be tightly regulated, since the presence of the gene on the multi-copy plasmid repressed the synthesis of the chromosomal gene. Cell lines carrying truncated 7SL RNA genes were established and their expression indicated that domain I is essential for expressing the 7SL RNA. No constructs carrying portions of the 7SL RNA were expressed, except for a construct which lacked 23 nt from the 3' end of the RNA. This suggests that 90% of the 7SL RNA molecule is important for its maintenance as a stable small RNA. We propose that the repression phenomenon may originate from a regulatory mechanism that coordinates the level of the 7SL RNA by its binding proteins.
Bibliographical noteFunding Information:
This work was supported by research grant from the Cemach and Anna Oiserman Research fund and from the Leo and Julia Forchheimer Center for Molecular Genetics of the Weizmann Institute. We wish to thank Ora Asher for excellent technical assistance.