DIRS-1 is a Dictyostelium discoideum transposable element that contains heat shock promoter sequences in the inverted terminal repeats. We showed that transcription of a 4.5-kilobase polyadenylated RNA initiates at a discrete site within the left-terminal repeat of DIRS-1, downstream from heat shock promoter and TATA box sequences. This RNA represents a full-length transcript of DIRS-1. We describe a cDNA clone that contains the 4.1 kilobases of internal sequence of DIRS-1, a cDNA clone that spans the junction between the internal sequences and the right-terminal repeat, and a cDNA clone that appears to have been transcribed from a rearranged genomic copy of DIRS-1. A second DIRS-1 RNA, named El, is transcribed on the opposite strand of DIRS-1 from the 4.5-kilobase RNA and is under control of the heat shock promoter in the right-terminal repeat. El transcription initiates at multiple positions both within and downstream from the right-terminal repeat. The same transcriptional initiation sites are used during normal development and during heat shock, suggesting that in all cases DIRS-1 transcription is regulated by the heat shock promoters contained within the two terminal repeats.Dictyostelium intermediate repeat sequence 1 (DIRS-1) is a moderately repetitive and apparently transposable element with several unusual features (4,7,15). DIRS-1 consists of 4.1 kilobases (kb) of unique internal sequence flanked by inverted terminal repeats of unequal length, 332 and 360 base pairs (bp) (6, 23). There are about 40 copies of the intact DIRS-1 element interspersed throughout the Dictyostelium genome, as well as an additional 200 copies of related sequences (7). Different Dictyostelium strains have different sequences flanking some of their DIRS-1 elements, indicating that the element had transposed after these strains were separated (7, 15).Transcription of DIRS-1-related sequences results in the production of a heterogeneous population of polyadenylated cytoplasmic RNAs that are differentially expressed during development (7, 24; C. Zuker, Ph.D. thesis, Massachusetts Institute of Technology, Cambridge). DIRS-1 RNAs are present at very low levels in growing cells, begin to accumulate within the first hour after initiation of development, and reach their maximal level by 15 h. Transcription of DIRS-1-related RNAs is induced in vegetative cells subjected to stresses such as heat shock and high cell density (22).Examination of the DNA sequence of the DIRS-1 terminal repeats has led to the identification of a putative heat shock promoter sequence (22, 23) homologous to the Drosophila consensus heat shock promoter (13,14). Stress-induced transcription of DIRS-1-related RNAs is presumed to be directed by these sequences. Cappello et al. (5) showed that a cloned partial copy of DIRS-1 (pB41.6) directs transcription of a heat-shock-inducible RNA in yeasts and that heatinducible transcription of this RNA is dependent on the presence of the DIRS-1 terminal repeat. The isolated terminal repeat also directs heat-inducible transcrip...