LOW RESOLUTION AB INITIO ENVELOPES OF YEAST RNA POLYMERASE II. David, P. R., Bushnell, D. A., Leuther, K. K., Subbiah, S., Bellamy, H. and Kornberg, R. D, Structural Biology Dept. Stanford University Medical Center MS 5400 Stanford, CA, 94305-5400 USA

Low resolution ab initio envelopes for yeast RNA polymerase II, an enzyme complex of 550,000MW, have been computed, using the condensing protocol methods. These envelopes show the packing within the unit cell and a strong similarity to the existing envelopes from two dimensional electron microscopy. The special methods used to obtain the lowest resolution reflections for such a large protein and assymmetric unit will be discussed. Despite the large size of the protein, envelopes can be calculated easily in a matter of minutes.