Organization of the Dictyostelium discoideum actin multigene family. Flanking sequences show subfamily homologies and unusual dyad symmetries.

Sequences flanking the protein-coding regions of 15 of the 17 to 20 actin genes in the cellular slime mold Dictyostelium discoidium have been determined. Comparison of sequences among genes shows that they contain extensive homologies at both the 5' and 3' ends of the coding regions. On the basis of these homologies, actin genes fall into three groups. Group I consists of Actin 8 alone. Group II consists of the two closely linked genes Actin 3-sub1 and Actin 3-sub2. These two genes differ from all other actin genes in the location of their TATA box and oligo(dT) run, and diverge substantially in their coding sequence as well. Group III contains all the rest of the genes we have studied. Within this group, there are two subgroups of genes, IIIA (Actins 5, 9 and 10) and IIIB (Actins M6, 2-sub1 and 2-sub2, 4, 6, 7, 11 and 12). Two actin cDNA clones, ITL-1 and III-12/A1, which have no cloned genomic counterparts, are members of groups IIIA and IIIB, respectively. Homologies at the 3' ends of genes do not extend beyond a short genomic poly(A) sequence, the probable termination of transcription. Homologies at the 5' ends may extend about 300 base-pairs 5' to the ATG but, in most cases, extend only about 150 base-pairs 5' to the ATG. We have identified a group of short, relatively G + C-rich sequences within the extremely A + T-rich sequence at the 5' ends of actin-coding regions, which are shared among different actin genes. Many of these sequences exhibit dyad symmetry, and their general location and order is conserved among the different actin genes. We suggest that they may have a role in regulation of the transcriptional patterns of individual actin genes.