ORDERED SPLICING OF THYMIDINE KINASE PRE-MESSENGER-RNA DURING THE S-PHASE OF THE CELL-CYCLE

Concomitant with the onset of S phase, a series of thymidine kinase (TK) splicing intermediates as well as mature TK mRNA accumulates in the nucleus of BALB/c 3T3 cells. Most of the TK splicing intermediates are retained by oligo(dT)-cellulose chromatography, and, therefore, 3' end formation and polyadenylation probably precede the splicing of TK pre-mRNAs. We have further characterized the TK pre-mRNAs that are present in the nuclei of S-phase cells by using specific probes derived from each of the six TK intervening sequences. Based on the sizes of the pre-mRNAs and their patterns of hybridization with these intron probes, we propose a pathway for intron removal from nascent TK transcripts. Intron excision occurred by a preferred, but not necessarily obligatory, order which appears to have been conserved in mouse and Chinese hamster cell.