Multiple, heterogeneous actin genes in Dictyostelium.

We have used an actin gene-containing restriction fragment of plasmid M6 (Kindle and Firtel, 1978) to select a second actin gene-containing plasmid which we have named pDd actin 2. This plasmid has been shown to contain two actin genes separated by 350 bp of nonactin DNA. When heteroduplexes are formed between any two of the three actin genes present in chimeric plasmids, the region of homology is 1100 +/- 100 bp. This is close to the minimum length required to code for actin protein. The 1100 bp region of intergene homology corresponds to the 1100 bp homology observed between M6 and the two actin cDNA plasmids pcDd actin B1 and pcDd actin A1 (Bender et al., 1978). We have no evidence for additional sequences common to either the 3' or 5' ends of the 1100 +/- 100 bp region of intergene homology. Thermal denaturation experiments show that different pairs of actin genes are diverged from each other by as much as 6--8%. There are two size classes of mRNA complementary to the three actin genes. These have lenghts of 1.25 and 1.35 kb as determined on methyl mercuric hydroxide-containing agarose gels. The possible linkage of these three actin genes to other actin genes is discussed.