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Carrier-free iodination of 5S RNA with (125)I yields a probe suitable for use in cytological RNA-DNA hybridization studies. The genes coding for 5S ribosomal RNA in Drosophila could be localized by autoradiography after a 2-day exposure, whereas a 2-month exposure is needed when the best available (3)HRNA is used. The procedure introduces one covalently bound (125)I atom per 100 nucleotides, resulting in a specific activity of over 10(8) dpm/mug of RNA. The method may be readily applied to other problems involving molecular hybridization techniques.
Prensky et al. (Fri,) studied this question.