## Abstract

An electron density map to 2.9 <latex>$\overset{\circ}{\mathrm A}$</latex> (1 <latex>$\overset{\circ}{\mathrm A}$</latex> = 10<latex>$^{-10}$</latex> m) resolution of Rubisco from Rhodospirillum rubrum has been obtained from crystals of the gene product expressed in Escherichia coli. These crystals are monoclinic, space group P2<latex>$_1$</latex> with cell dimensions a = 65.5 <latex>$\overset{\circ}{\mathrm A}$</latex>, b = 70.6 <latex>$\overset{\circ}{\mathrm A}$</latex>, c = 104.1 <latex>$\overset{\circ}{\mathrm A}$</latex> and <latex>$\beta$</latex> = 92<latex>$^\circ$</latex> and with two subunits per asymmetric unit. Isomorphous phases were obtained from three heavy atom derivatives. The dimeric molecule has the shape of a distorted ellipsoid with approximate dimensions 45 x 70 x 105 <latex>$\overset{\circ}{\mathrm A}$</latex>. The subunit interactions in the dimeric molecule are tight and extensive. Each subunit is divided into two main domains, one of which has extensive <latex>$\alpha/\beta$</latex> structure probably folded into the common eight unit barrel structure. NADPH and pyridoxal phosphate bind at one end of this domain in each subunit. Two different octameric molecules with approximate (422) symmetry have been constructed from the L<latex>$_2$</latex> dimeric molecule from R. rubrum, assuming that the molecular twofold axis of the L<latex>$_2$</latex> dimer is preserved in the L<latex>$_8$</latex> molecule. It is shown that only one of these is compatible with the orthorhombic crystal lattice of spinach and Alcaligenes eutrophus Rubisco. The L<latex>$_8$</latex> molecule thus obtained is ellipsoidal with approximate dimensions 130 <latex>$\overset{\circ}{\mathrm A}$</latex> x 130 <latex>$\overset{\circ}{\mathrm A}$</latex> x 105 <latex>$\overset{\circ}{\mathrm A}$</latex> and with a large hole in the middle of about 40 <latex>$\overset{\circ}{\mathrm A}$</latex> diameter around the fourfold axis. A cleft in each subunit between the two domains could form a binding site for the small subunit in the plant L<latex>$_8$</latex>S<latex>$_8$</latex> Rubisco molecule.