Putative actin genes in the macronucleus of Oxytricha fallax.

Abstract

The macronuclear DNA of the hypotrichous ciliate, O. fallax is arranged as short achromosomal pieces, 22-0.5 kbase pairs (kb) in length. Micronuclear DNA has a typical chromosomal organization. Macronuclear DNA is derived from micronuclear DNA through a process of polytene chromosome fragmentation, with a resultant decrease in DNA sequence complexity. Three putative actin genes were identified in macronuclear DNA by using a cloned yeast actin gene as a hybridization probe. A restriction fragment of the yeast gene containing both actin coding and noncoding DNA hybridizes strongly to 2 macronuclear DNA pieces, 1.6-1.4 kb in length and weakly to a 1.2-kb piece. The entire 1.6 kb piece was cloned in plasmid pBR322 and the resulting recombinant plasmid was designated pOfACT(1.6). The 1.6 kb pOfACT(1.6) insert hybridizes only to those restriction fragments of the yeast actin gene containing actin coding sequences. When hybridized to macronuclear DNA under conditions that allow the yeast probe to hybridize to all 3 macronuclear pieces, the pOfACT(1.6) insert hybridizes only to the 1.6 kb piece. Under less stringent conditions the insert also hybridizes to the 1.4 kb piece, but it shows no hybridization to the 1.2 kb DNA. The macronuclear pieces homologous to the yeast actin gene thus differ in sequence and are interpreted as a related family of actin genes. Each of these pieces could accommodate an actin coding sequence, which in yeast, Dictyostelium discoideum and Drosophila melanogaster is 1.1 kb, and an additional 0.1-0.5 kb of noncoding DNA.