Organization of Actin Gene Sequences in the Sea Urchin: Molecular Cloning of an Intron-Containing DNA Sequence Coding for a Cytoplasmic Actin

Southern transfer and solution hybridization experiments, using as probe a DNA fragment that encodes for Drosophila actin, demonstrate cross hybridization to DNA from the sea urchin Strongylocentrotus purpuratus. Recombinant DNA clones that contained sea urchin genomic DNA fragments were constructed and screened for the presence of actin-encoding DNA sequences by colony hybridization with the Drosophila actin sequence. Two different putative actin-encoding clones were identified and were shown to specifically hybridize actin-encoding mRNA from a complex mRNA population. Southern blot hybridization experiments with both the Drosophila actin sequence and one of the cloned sea urchin sequences, in conjunction with solution hybridization data, suggest an actin gene copy number of 5-20 per haploid genome. DNA sequence analysis of one of the cloned sequences indicates that this fragment codes for a cytoplasmic form of actin and contains an intervening sequence of at least 200 nucleotides beginning immediately after amino acid 121 in the protein sequence.