S U M M A R YCowpea mild mottle virus (CMMV), a previously undescribed virus widespread in cowpeas (Vigna unguiculata) in the Eastern Region of Ghana, was seed-borne in K unguiculata, Phaseolus mlgaris and Glycim max, but was not transmitted by twelve aphid species including Aphis craccivora, A . fabae, Acyrthosiphon pisum and Myzus pusicae. CMMV was transmitted by inoculation of sap to eleven of seventeen members of the Papilionaceae causing very severe diseases in G. max and Arachis hypogaea, and to ten of fifty-one species within five of nineteen other families; it was best propagated in G. max and Nicotiana clevelandii, and assayed in Chenopodium quinoa. Sap from systemically infected G . m m was infective after dilution to 109 but not I O -~, after 10 min at 65 "C but not at 70 "C, or after 4 days at 18 "C or 16 days at 2 "C. Lyophilized sap was infective after 3 years in vacuo.CMMV has straight to slightly flexuous, fragile filamentous particles, c. 13 x 650 n m which, in sap, are occasionally surrounded by a loose external spiral. About 5 mg of purified virus was obtained from I kg of leaf tissue of G . max or N . clevelandii by clarifying leaf extracts in 0.02 M borate buffer (pH 9 -5 ) with chloroform, followed by two or three cycles of differential centrifugation, and density gradient centrifugation. Virus preparations had ultraviolet absorption spectra typical of a nucleoprotein containing c. 5 yo nucleic acid, contained numerous particles without external spirals, which sedimented as a single component with a sedimentation coefficient SO^,,^) of 165 z 4S, and contained a single polypeptide species with a molecular weight of 32 00-33 000.
J. BURCR I955 3. The relation between the dye concentration and the apparent value of the Michaelis constant is not linear. It resembles the function entailed by the 'apparent competitive' inhibition mechanism of Segal et al. (1952), but on analysis is found to deviate significantly from it. 4. An inhibition mechanism is proposed which accounts for the experimental results, including the relation between the inhibitions of the hydrolyses of two different substrates. 5. Distribution experiments and freezing-point measurements show that rhodamine B is monomeric in the aqueous solutions of pH 7-4 used for the enzyme studies. 6. Measurements of the polarization of fluorescence demonstrate the reversible combination of rhodamine with large molecules in the enzyme preparation. The combination is accompanied by a change in absorption spectrum and an increase in fluorescent intensity. 7. Displacement of the dye from combination by ethyl acetate gives evidence that the binding is specifically related to the esterase inhibition. 8. The fluorescence data are analysed quantitatively, and their relation to the inhibition results is discussed. The author is deeply grateful to Dr G. Weber for introducing her to the fluorescence method, putting his apparatus at her disposal, and providing much assistance by discussions during the course of this work.
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