The 1017-bp intron within the cloned phage T4 td gene was deleted by oligonucleotide-directed mutagenesis. Induction of thymidylate synthase activity and mature td mRNA from this intronless construct (pKTd delta I) was compared both in vivo and in vitro with expression from plasmids bearing td genes in which the introns contain either no change (pKTd2), an XbaI linker inserted about 200 nucleotides from the 3'-end (pKTdX-1), or a deletion of two-thirds of the central portion (pKTd delta 1-3). Slightly more synthase accumulated in cells carrying pKTd delta I as compared to the other td genes when induction was performed at 30, 37, or 42 degrees C. Dramatically different results were observed in vitro, where enzyme activity synthesized from pKTd delta I DNA appeared earlier and reached severalfold higher levels than with pKTd2 DNA. In addition, thymidylate synthase expression from pKTdX-1 was impaired relative to pKTd2, while pKTd delta 1-3 accumulated enzyme at levels intermediate between those of pKTd2 and pKTd delta I. Under both in vivo and in vitro conditions, increasing levels of mature td mRNA preceded and paralleled those in enzyme activity for all four plasmids, demonstrating comparable translation of the mRNAs produced. From these results it would appear that the splicing of td RNA is much more efficient in vivo than in vitro, suggesting that other cellular components may facilitate in vivo processing of this intron-containing transcript.