The present proposal is designed to provide information on the cellular regulatory mechanisms which coordinate the synthesis of phospholipids with membrane assembly. The proposed investigations specifically focus upon the regulatory systems which govern the production of phospholipids in phototrophically and chemotrophically growing cells of Rhodopseudomonas sphaeroides. The major objective will be to establish a direct causal relationship between the physical state of the membrane and the rate and extent of phospholipid production. It is proposed that changes in the membrane physical state induced by fluctuations in the relative content of membrane proteins function in a direct, autoregulatory manner to control phospholipid synthesis by membrane associated enzymes. It is further proposed that the site of this regulation is the sn-glycerol-3-phosphate acyltransferase; the enzyme that catalyzes the first committed step in bacterial phospholipid synthesis. Initially, investigations will focus upon the in vitro characterization and membrane localization of sn-glycerol-3-phosphate acyltransferase activity in chemotrophically and phototrophically grown cells. By employing a variety of physiological growth regimens, correlations will be made between the rates of cellular phospholipid synthesis and compositional modifications of the membrane fractions shown to possess enzyme activity. These studies will be accompanied by direct, in vitro measurements of enzyme activity in membrane preparations in which the relative content of membrane protein has been either physiologically, or artifically, modified. Studies employing differential scanning calorimetry will be employed to directly determine the influence of membrane protein content on the membrane physical state.