The architecture of unusual membrane tubes in the B800-850 light-harvesting bacteriochlorophyll-deficient mutant 19 of Rhodobacter sphaeroides

The B800-850 light-harvesting bacteriochlorophyll-deficient mutant 19 of Rhodobacter sphaeroides exhibits in ultrathin sections not only regularly shaped intracytoplasmic membrane (ICM) vesicles (diameter of 43 nm), but also long tubes of ICM with diameters ranging from 60 to 95 nm. Freeze-fracture preparations revealed that these tubes were densely covered with intramembrane particles of 10.4 nm in diameter. According to the ratio of protein to phospholipid as calculated for the ICM tubes of mutant 19 these tubes do not represent lipid-enriched ICM-invaginations as discussed for other mutants of R. sphaeroides with similar tubular ICM. A comparable particle arrangement was found on the plasmic fracture face of the cytoplasmic membrane in wild-type cells of R. sphaeroides. The architecture of the ICM in the B800-850-deficient mutant Ala+ of the closely related R. capsulatus exhibited, in comparison to the wild-type strain, significant differences in the number of particles per-mu-m2 and in the ratio of protein to phospholipid but no significant differences in the size of the intramembrane particles.