Topological Forms of Cauliflower Mosaic Virus Nucleic Acid

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Abstract
The nature of the topological forms of the genome of cauliflower mosaic virus (CauMV) in infectious nucleic acid (NA) preparations was examined by velocity sedimentation in sucrose density gradients, by equilibrium sedimentation in CsCl and CsCl-ethidium bromide (EthBr) density gradients, by electrophoresis in composite polyacrylamide-agarose gels and by EM. The distribution of CauMV-NA in centrifuged sucrose density gradients is heterogeneous and CauMV-NA is susceptible to degradation by alkali (0.03 N and 0.3 N NaOH). The alkali-sensitive (0.3 N NaOH) sites may be located at specific, widely separated sites in the genome. Molecules of CauMV-NA are linear and circular. The circular forms of CauMV-NA are twisted and non-twisted. The twisted circular structures are the predominant forms in rapidly sedimenting fractions from centrifuged sucrose density gradients and in the slowest of 3 distinct electrophoretic components in composite polyacrylamide-agarose gels. The untwisted circles may be nicked or relaxed forms of an otherwise closed duplex. No covalently closed circular NA was detected by centrifugation in alkaline sucrose or in CsCl-EthBr density gradients. Topological constraints on the NA other than covalent closure of the strands might be responsible for these forms. The twisted circles appear to be stable and distinct from the untwisted, open circles. No linear or circular oligomeric forms of the monomeric unit length (2.3-2.5 .mu.m) circles were observed. Infectivity was associated only with fractions containing the twisted circular forms of CauMV-NA from centrifuged sucrose density gradients.

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