The modes of synthesis of cell envelope components in Escherichia coli.

In an attempt to elucidate the patterns of synthesis of some envelope components during the bacterial division cycle, the rate of synthesis of phospholipid and envelope proteins has been determined throughout the cell cycle of E.coli B/r. To facilitate this study, a convenient method of preparation of envelopes was devised which leads to the recovery of approximately half the membranous material of the bacterial envelope. Further characterisation of this material indicates that it is derived randomly from, and is therefore representative of the whole bacterial envelope. In the first instance the membrane elution technique devised by Helmstetter (1967) was employed, and the following results were obtained. The rate of phospholipid synthesis was found to increase continuously and exponentially throughout the division cycle, whilst the rate of envelope protein synthesis was found to be constant throughout part of the cycle which doubled in rate at a discrete time. Further fractionation of envelope proteins by SDS-PAGE showed that the rate of synthesis of approximately half the envelope proteins increased two-fold, whilst that of the other half increased three-fold. Similar results were obtained using synchronous cultures except that all envelope proteins showed a two-fold increase in rate of synthesis. Pulse-chase experiments performed with synchronous cultures showed that the class of proteins that underwent an apparent three-fold increase in rate of synthesis were also subsequently lost from the envelope at a greatly increased rate. Combining the results of all three sets of experiments it is concluded that, whilst the rate of synthesis of phospholipid increased continuously throughout the division cycle, the rate of envelope protein synthesis remained constant with discrete doublings in rate of synthesis. Some envelope proteins are lost from the envelope, probably also at a discrete time in the division cycle. The experiments performed with synchronous cultures demonstrated the appearance of a protein synthesised at about the time of termination of rounds of replication, which was only transiently associated with the bacterial envelope. Furthermore, the synthesis of this protein was greatly stimulated by inhibition of DNA synthesis caused by removal of thymine from the growth medium. The role this protein plays in the bacterial cell remains unknown.