GENETIC BEHAVIOR OF A CLONED MOUSE RIBOSOMAL DNA SEGMENT MIMICS MOUSE RIBOSOMAL GENE EVOLUTION

A 1.7 .times. 103 base pair [bp] SalI fragment of mouse ribosomal gene spacer undergoes recA-independent deletions of DNA in units of approximately 126 bp when cloned in .lambda. bacteriophage or bacterial plasmids. The structure of the 1.7 .times. 103 bp piece was examined with PvuII; it was composed of about equal numbers of copies of each of 2 subrepeating units, 120 and 130 bp in size. The correlation between the size of the structural subunits and the functional genetic unit of this fragment as expressed in E. coli led to the study of the organization of these sequences in mice. SalI (or HindII) digests of DNA samples from wild and inbred strains revealed extensive heterogeneity in the size of fragments homologous to this 1.7 .times. 103 bp piece. A total of 15 different size classes were detected in the samples. These fragments were also organized in PvuII repeating units about equal in size to the PvuII repeats in the cloned 1.7 .times. 103 bp piece. Using least squares estimation to determine the repeat length of DNA it was determined that the 15 different fragments found in the mouse DNA samples probably originated as a result of genetic events based on a 135 bp structural unit. The similarity between the size of the PvuII structural unit and the unit of genetic behavior in both the cloned and uncloned DNA samples was probably significant. There are aspects of the nucleotide structure or organization of the PvuII repeating units that play a dominant role in its genetic behavior, regardless of whether these sequences are present in E. coli or mice. The clones containing this mouse sequence may provide an experimental system for studying the nature of the genetic events that are involved in multigene evolution.