Conserved sequences in the U2 snRNA-encoding genes of Kinetoplastida do not include the putative branchpoint...

The U2 small nuclear RNA (snRNA) of Trypanosoma brucei gambiense, a flagellated protozoon of the order Kinetoplastida, is 148 nucleotides (nt) long, and thus the smallest U2 snRNA identified so far. To examine the evolutionary conservation of this RNA among Kinetoplastida, we have cloned and sequenced the U2 genes from Trypanosoma congolense and Leishmania mexicana amazonensis, which are 145 and 141 nt in length, respectively. The sequences of the Kinetoplastida U2 snRNAs are essentially identical in the 5' half of the molecule. Surprisingly, the putative branch site recognition sequence of L. m. amazonensis U2 snRNA shows two nt changes when compared with the other two U2 snRNAs. The sequence of the 3' half of the Kinetoplastida U2 snRNAs is less conserved with T. congolense and L. m. amazonensis RNAs showing 23 and 35 nt sequence variations, respectively, when compared with the corresponding sequence of the T. b. gambiense U2 snRNA. Alignment of the flanking regions of the U2 genes revealed several elements which are conserved both in sequence and in position relative to the U2 coding region and which may function in the biosynthesis of U2 snRNAs. One upstream element specifically binds protein factor(s) present in T. brucei nuclear extracts.