Deeming and B. L. Shaw, ibid., 2705 (1970). ( 12) The rate constant for reaction of Ir(CO)(PMe2Ph)«Cl with H2 (in the absence of added PMe2Ph) increases by only a factor of two (from 0.36 to 0.68 M-1 sec-1) in going from chlorobenzene to DMF. Comparably small solvent effects have been found for reactions of H2 with Ir(CO)(PPh3)2Cl and with other Ir1 complexes.4

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The sedimentation properties of ribonucleic acid (RNA) polymerase-polydeoxynucleotide complexes have been examined by analytical and density gradient centrifugation. Whereas free RNA polymerase from Escherichia coli had a sedimentation coefficient of about 24 S under the conditions utilized, when the enzyme was complexed with short polydeoxynucleotides its sedimentation coefficient was decreased. When the polynucleotide to enzyme mole ratio was about 0.4, a peak sedimenting at 19 S appeared in addition to T he central role of RNA polymerase in the transfer of biological information stimulates interest in the structure and catalytic mechanism of this enzyme. We have begun studies aimed to help provide such information for RNA polymerase as isolated from Escherichia coli (Smith et al., 1966(Smith et al., , 1967. It is now generally agreed that RNA polymerase freshly isolated from E. co/i is a large enzyme which has a sedimentation coefficient of 21-24 S in low ionic strength buffers. It undergoes complex dissociation-reassociation reactions upon raising the ionic strength or pH, or upon aging (Fuchs et a/., 1964; Colvlll ef a/., 1966; Richardson, 1966a; Stevens ef al., 1966;. Thus the question of the size of the active enzyme molecule is raised. Fuchs et al. (1964) suggested that the 24s form of the enzyme was the "native" form. Richardson (1 966a, b) also favored the large form, since he found that under ionic conditions which favor dissociation no catalytic activity could be detected. However, our experiments indicated that under these ionic conditions RNA polymerase activity might or might not be demonstrable, depending upon the particular template in use . In order to obtain more direct evidence regarding the size of the active RNA polymerase molecule, we have investigated the sedimentation properties of complexes of the enzyme and small polynucleotides. Materials and MethodsMaterials. RNA polymerase was isolated as previously described and stored under liquid nitrogen. The specific activity of the preparations

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