Cytology and Morphogenesis of the Prokaryotic Cell

Abstract

Internal membranes observed in the methanogenic bacterium Methanobacterium thermoautotrophicum (Zeikus and Wolfe 1973; Doddema et al. 1979; Sauer et al. 1980; Sprott et al. 1984) had led to speculations about their physiological function. Kell et al. (1981) proposed that the internal membrane systems form an autonomous organelle, which is the site of methanogenesis and ATP synthesis. The term “methanochondrion” was coined by these authors for this “organelle”, in analogy with the bioenergetic function of mitochondria. The existence of this compartment was based on structural studies (see above), on experiments on the effect of uncouplers on pH-gradient and/or membrane potential and on methanogenesis and ATP synthesis (Doddema et al. 1978; Sauer et al. 1981; Schönheit and Beimborn 1985), on investigations on the orientation of the ATPase molecules (Doddema et al. 1979), and on experiments on the transport of adenine nucleotides across internal membranes (Doodema et al. 1979, 1980). Recently, Aldrich et al. (1987a) demonstrated that these internal membranes are preparation artifacts. They reported that cells fixed in phosphate-buffered 2.5% glutaraldehyde, 0.1% osmium tetroxide, or 0.5% glutaraldehyde — 2.5% formaldehyde always contained internal membranes. However, cells freeze-fractured, freeze-substituted, or fixed in cacodylate-buffered 1% osmium tetroxide, 2.5% glutaraldehyde, or simultaneous glutaraldehyde-osmium lacked internal membranes. Therefore, it can be concluded that these structures are preparation artifacts. As a consequence, in order to test the “methanochondrion concept”, Krämer and Schönheit (1987) studied the uptake of adenine nucleotides in various membrane preparations of M. thermoautotrophicum. These authors ruled out an exchange mechanism of the mitochondrial type, and a general exchange of a uniport mechanism. The “nucleotide uptake” was shown to be, in fact, a tight and specific binding of ADP and ATP to binding sites at the interior of the cell membrane. In protoplasts which were shown not to contain internal membranes, also nucleotide binding was observed; however, this phenomenon only occurred after disruption of the cytoplasmic membrane by osmotic lysis, thus exposing the binding sites.