SEQUENCES WITHIN THE 5'-UNTRANSLATED REGION REGULATE THE LEVELS OF A KINETOPLAST DNA TOPOISOMERASE MESSENGER-RNA DURING THE CELL-CYCLE

Gene expression in trypanosomatids appears to be regulated largely at the posttranscriptional level and involves maturation of mRNA precurso rs by trans splicing of a 39-nucleotide miniexon sequence to the 5' en d of the mRNA and cleavage and polyadenylation at the 3' end of the mR NA. To initiate the identification of sequences involved in the period ic expression of DNA replication genes in trypanosomatids, we have map ped splice acceptor sites in the 5' flanking region of the TOP2 gene, which encodes the kinetoplast DNA topoisomerase, and have carried out deletion analysis of this region on a plasmid-encoded TOP2 gene. Block delections within the 5' untranslated region (UTR) identified two reg ions (-608 to -388 and -387 to -186) responsible for periodic accumula tion of the mRNA. Deletion of one or the other of these sequences had no effect on periodic expression of the mRNA, while deletion of both r egions resulted in constitutive expression of the mRNA throughout the cell cycle. Subcloning of these sequences into the 5' UTR of a constru ct lacking both regions of the TOP2 5' UTR has shown that an octamer c onsensus sequence present in the 5' UTR of the TOP2, RPA1, and DHFR-TS mRNAs is required for normal cycling of the TOP2 mRNA. Mutation of th e consensus octamer sequence in the TOP2 5' UTR in a plasmid construct containing only a single consensus octamer and that shows normal cycl ing of the plasmid-encoded TOP2 mRNA resulted in substantial reduction of the cycling of the mRNA level. These results imply a negative regu lation of TOP2 mRNA during the cell cycle by a mechanism involving red undant elements containing one or more copies of a conserved octamer s equence within the 5' UTR of TOP2 mRNA.