Histone genes in macronuclear DNA of the ciliate Stylonychia mytilus

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Abstract
DNA in the macronucleus of Stylonychia mytilus exists as discrete gene-sized fragments which are derived from micronuclear DNA through a series of well-defined developmental events. It has been proposed that each of the DNA fragments might represent a gene and its controlling elements. We have investigated this possibility using genes which code for the five histone proteins. Macronuclear DNA fragments were fractionated according to size by agarose gel electrophoresis, the fragments transferred to nitrocellulose filters using the technique of Southern, and the filter-bound DNA hybridized with labeled cloned histone genes of the sea urchin, Psammechinus miliaris. Results indicate, first, that sequences homologous to the five individual histone gene probes are present in discrete macronuclear fragments which appear as bands in the gel hybridization assay. Secondly, for each of the five individual histone gene probes the homologous DNA fragments are several in number, ranging in size from 7.6 Kb (Kilo base pairs) to 0.73 Kb. For example, the largest of six detected fragments hybridizing to the H3 gene probe contains approximately 10 times the amount of DNA required to code for a Stylonychia H3 histone. The smallest detected fragment hybridizing to the H3 probe contains enough DNA to code for approximately two copies of the histone. Finally, in general, no two histone gene probes hybridized to the same macronuclear DNA fragment. This result indicates that genes coding for the five histones in Stylonychia are not located together on the same macronuclear DNA fragments and implies that the five functionally related genes would not be transcribed together as a polycistronic unit.