The actin genes of Drosophila: protein coding regions are highly conserved but intron positions are not.

The entire set of six closely related Drosophila actin genes was isolated using recombinant DNA methodology, and the structures of the respective coding regions were characterized by gene mapping techniques and by nucleotide sequencing of selected portions. Structural comparisons of these genes have resulted in several unexpected findings. Most striking is the nonconservation of the positions of intervening sequences within the protein-encoding regions of these genes. One of the Drosophila actin genes, DmA4, is split within a glycine codon at position 13; none of the remaining five genes is interrupted in the analogous position. Another gene, DmA6, is split within a glycine codon at position 307; at least two of the Drosophila actin genes are not split in the analogous position. Additionally, none of the Drosophila actin genes is split within codon four, where the yeast actin gene is interrupted. The six Drosophila actin genes encode several different proteins, but the amino acid sequence of each is similar to that of vertebrate cytoplasmic actins. None of the genes encodes a protein comparable in primary sequence to vertebrate skeletal muscle actin. Surprisingly, in each of these derived actin amino acid sequences in the initiator methionine is directly followed by a cysteine residue, which in turn precedes the string of three acidic amino acids characteristic of the amino termini of mature vertebrate cytoplasmic actins. We discuss these findings in the context of actin gene evolution and function.