Localization, multiplicity, and repeat size of the 5S rRNA genes

In 1974 Wimber et al. (Chromosoma 47:353-359) reported that the maize 5S rRNA genes probably are located in 2L. In 1979 (Genetics 91:S76) we confirmed the 5S rRNA gene location. With two homozygous interchange lines, T2-6(8786) (6SNOR, 2S.97) and T3-6(030-8) (6SNOR, 3S.05) the 5S rRNA sequences hybridized to a site in 2L. Steffensen and Patterson (Genetics 91:S123, 1979) used the same approach to confirm the location of the 5S genes. Using a heterozygous interchange stock T2-6(5419) (6SNOR, 2L.82), they showed that the 5S genes are beyond 0.82 in 2L. Independently we performed in situ hybridization with T2-6(5419) but in homozygous condition. Our results place the genes at 2L.88. Only one site of silver grain localization has been observed in the various strains.

Using filter DNA/5S rRNA saturation hybridization we estimate 12,000 5S rRNA genes/2c nucleus in the inbred A188. For comparison, A188 possesses 7000 18 and 28S rRNA genes/2c nucleus.

We have begun to characterize the 5S rDNA repeat unit using restriction endonucleases. When maize DNA is digested to completion with the enzyme Bam H1 a basic unit occurs of 310 nucleotides. Dimers and multimers up to 10 times the size of the basic repeat unit are also clearly seen in these digests. This suggests that either the 5S genes have undergone considerable divergence or that the bases are modified rendering them unrecognizable by the enzyme. A more detailed report on this system is in preparation.

P. N. Mascia, R. L. Phillips, A. S. Wang and I. Rubenstein

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