The Escherichia coli cell cycle

This review summarizes present knowledge of the bacterial cell cycle with particular emphasis on Escherichia coli. We discuss data coming from three different types of approaches to the study of cell extension and division: The search for discrete events occurring once per division cycle. It is generally agreed that the initiation and termination of DNA replication and cell septation are discrete events; there is less agreement on the sudden doubling in rate of cell surface extension, murein biosynthesis and the synthesis of membrane proteins and phospholipids. We discuss what is known about the temporal relationship amongst the various cyclic events studied. The search for discrete growth zones in the cell envelope layers. We discuss conflicting reports on the existence of murein growth zones and protein insertion sites in the inner and outer membranes. Elucidation of the mechanism regulating the initiation of DNA replication. The concept of "critical initiation mass" is examined. We review data suggesting that the DNA is attached to the envelope and discuss the role of the latter in the initiation of DNA replication.     

Variations in some molecular events during the early phases of the Reuber H35 cell cycle. IV-regulation of tyrosine aminotransferase

Tyrosine aminotransferase activity increased during conversion of serum depleted quiescent Reuber H35 rat hepatoma cells into the proliferative state. Increased activity coincides with the actual increase of cells into S phase. The rate of tyrosine aminotransferase synthesis along the cell cycle was studied. The rate of enzyme synthesis fluctuated through the cell cycle but could not explain the increase of specific activity. Apparently enzyme activity is predominantly regulated by a post-translational event. Intracellular levels of cyclic AMP and cyclic GMP were measured at various times of G1 and S phases. In the early part of the cell cycle tyrosine aminotransferase decreased while intracellular levels of cyclic AMP increased. At later stages cyclic AMP rises concurrently with increased rates of enzyme synthesis. Induction of tyrosine aminotransferase by N6,O2'-dibutyryladenosine 3', 5'-monophosphate (Bt2cAMP) was studied. Inducibility by Bt2cAMP fluctuated through the cell cycle. Alternation of positive and negative control of tyrosine aminotransferase synthesis was observed. In early serum induced cells, Bt2cAMP increased enzyme activity without any increased rate of enzyme synthesis, on the contrary, a decreased rate of synthesis was observed. The data support the view that alternation of positive and negative control of tyrosine aminotransferase synthesis and temporary post-translational control of enzyme activity determine the enzyme level during the transition of quiescent hepatoma cells into proliferation.     

Variations in some molecular events during the early phases of the Reuber H 35 cell cycle. II.-Chromatin protein phosphorylation and protein kinases

Reuber H 35 hepatoma cells were synchronized by transfer in a serum free medium. Growth was re-initiated by addition of serum. Under these conditions DNA synthesis exhibited a maximum after 24 hours. Chromatin non-histone proteins prepared from cells at various phases of the cell cycle were incubated with [gamma-32P] ATP and the radioactive pattern of protein bound 32P was analysed by electrophoresis on polyacrylamide gels. No radioactive peak was observed in G0. Several peaks appeared 3 hours after the addition of serum. The radioactivity progressively increased until the cells reached the S phase. When most of the cells were in the S phase the radioactivity strongly decreased. Chromatin protein kinase activities were found to increase in late G1 and continued to increase in the S phase. The increase was 65% when phosvitin was the substrate, 100% with casein and histone H1. It is suggested that chromatin phosphorylated proteins could be involved in the mechanism which initiates DNA synthesis in G1 phase cells.     

Variations in some molecular events during the early phases of the reuber H 35 hepatoma cell cycle. I. Glucocorticoid induction of tyrosine aminotransferase.

1. Reuber H 35 hepatoma cell cultures were syncrhonized by serum depletion of the growth medium for 72 hr, which results in arrest of the cells in the G0 or G1 phase of the cell cycle. 2. Induction of tyrosine aminotransferase by dexamethasone was studied. Induction along the cell cycle varies with respect to the sensitivity of the cell towards low hormone concentration and the maximum effect elicited by the hormone. 3. Scatchard analyses of receptor- [3H]triamcinolone binding was performed in cell extracts prepared from cells at various times of G1 and S. Variations were observed in the concentration of glucocorticoid receptor as well as in the affinity of the receptor for the hormone. 4. During the latter part of the cell cycle, variations in the concentrations of the receptor could not explain the variation in enzyme induction, since the maximum rate of induction decreased while an increase in receptor activity still occurred.     

Suppression of cell proliferation and deoxyribonucleic acid synthesis in the cloned rat hepatoma H4‐II‐E cells overexpressing regucalcin

The role of endogenous regucalcin (RC) in the regulation of cell proliferation was investigated in the cloned rat hepatoma H4‐II‐E cells overexpressing RC stably. H4‐II‐E cells were transfected with RC/pCXN2 vector and the multiple neomycin‐resistant clones which overexpress stably RC were selected. The RC content of RC/pCXN2‐transfected cells used in this study was 19.7‐fold as compared with that of the parental wild type H4‐II‐E cells. Wild type H4‐II‐E cells, pCXN2 vector‐transfected cells (mock type), and RC/pCXN2‐transfected cells (transfectants) were cultured for 24, 48, and 72 h in the presence of fetal bovine serum (10% FBS). Cell numbers of wild and mock type were significantly increased with the time course of culture. Cell numbers of transfectants was significantly suppressed as compared with that of wild and mock type. Deoxyribonucleic acid (DNA) synthesis activity in the nuclear fraction of H4‐II‐E cells was significantly suppressed in transfectants with culture for 12–48 h. The presence of anti‐RC monoclonal antibody (10–50 ng/ml) in the reaction mixture caused a significant increase in DNA synthesis activity in the nuclei of wild type and transfectants; this increase was remarkable in transfectants. The effect of anti‐RC monoclonal antibody (50 ng/ml) in increasing DNA synthesis activity in transfectants was completely prevented by the addition of regucalcin (1 μM). This study demonstrates that cell proliferation is suppressed in the cloned rat hepatoma H4‐II‐E overexpressing RC stably. J. Cell. Biochem. 84: 143–149, 2002. © 2001 Wiley‐Liss, Inc.