FORMATION OF 2',3'-CYCLIC PHOSPHATES AT THE 3'-END OF HUMAN U6 SMALL NUCLEAR-RNA IN-VITRO - IDENTIFICATION OF 2',3'-CYCLIC PHOSPHATES AT THE 3'-ENDS OF HUMAN SIGNAL RECOGNITION PARTICLE AND MITOCHONDRIAL RNA PROCESSING RNAS

Approximately 90% of human U6 small nuclear RNA (snRNA) contains uridi ne cyclic phosphate (U > p) at its 3'-end (Lund, E., and Dahlberg, J. E. (1992) Science 255, 327-330). We studied the formation of U > p at the 3' end of human U6 snRNA using an in vitro system where uridylic a cid residues are added from UTP precursor and U > p is formed. Analysi s of U6 snRNAs with varying number of uridylic acid residues showed th at each of these species contains U > p where the phosphate originated from cx-phosphate of UTP precursor, The cyclic phosphate formation oc curred on U6 snRNA in extracts where essential spliceosomal snRNAs wer e specifically degraded, thereby indicating that U > p formation is no t coupled to pre-mRNA splicing, A subpopulation of human signal recogn ition particle and mitochondrial RNA processing RNAs isolated from HeL a cells also contained cyclic phosphates at their 3' ends. These data suggest that U > p in U6 snRNA is unlikely to be related to its partic ipation in splicing of pre-mRNAs, It appears that cyclic phosphate is an intermediate product in the metabolism of these small RNAs.