Structure and function of a chloroplast DNA replication origin of Chlamydomonas reinhardtii

Chloroplast DNA replication in Chlamydomonas reinhardtii is initiated by the formation of a displacement loop (D-loop) at a specific site. One D-loop site with its flanking sequence was cloned in recombinant plasmids SC3-1 and R-13. The sequence of the chloroplast DNA insert in SC3-1, which includes the 0.42-kilobase (kb) D-loop region, as well as 0.2 kb to the 5' end and 0.43 kb to the 3' end of the D-loop region, was determined. The sequence is A+T-rich and contains four large stem-loop stuctures. An open reading frame potentially coding for a polypeptide of 136 amino acids was detected in the D-loop region. One stem-loop structure and two back-to-back prokaryotic-type promoters were mapped within the open reading frame. The 5.5-kb EcoRI fragment cloned in R-13 contains the 1.05-kb SC3-1 insert and its flanking regions. A yeast autonomously replicating (ARS) sequence and an ARC sequence, which promotes autonomous replication in Chlamydomonas, have been mapped within the flanking regions [Vallet, J.-M. & Rochaix, J.-D. (1985) Curr. Genet. 9, 321-324]. Both R-13 and SC3-1 were active as templates in a crude algal preparation that supports DNA synthesis. In this in vitro system, chloroplast DNA synthesis initiated near the D-loop site.

Long-lasting decrease in neuronal Ca2+/calmodulin-dependent protein kinase II activity in a hippocampal neuronal culture model of spontaneous recurrent seizures

Ca2+/calmodulin-dependent protein kinase II (CaM Kinase II) activity was evaluated in a well-characterized in vitro model of epileptiform activity. Long-lasting spontaneous recurrent seizure (SRS) activity was induced in hippocampal neuronal cultures by exposure to low Mg2+ media for 3 h. Analysis of endogenous Ca2+/calmodulin-dependent phosphorylation revealed a significant long-lasting decrease in 32P incorporation into the alpha (50 kDa) and beta (60 kDa) subunits of CaM kinase II in association with the induction of SRS activity in this preparation. Ca2+/calmodulin-dependent substrate phosphorylation of the synthetic peptides, Autocamtide-2 and Syntide II, was also significantly reduced following the induction of SRSs and persisted for the life of the neurons in culture. The decrement in CaM kinase II activity associated with low Mg2+ treatment remained significantly decreased when values were corrected for changes in levels of alpha subunit immunoreactivity and neuronal cell loss. Addition of the protein phosphatase inhibitors, okadaic acid and cyclosporin A, to the phosphorylation reaction did not block the SRS-associated decrease in substrate phosphorylation, indicating that enhanced phosphatase activity was not a contributing factor to the observed decrease in phosphate incorporation. The findings of this study demonstrate that CaM kinase II activity is decreased in association with epileptogenesis observed in these hippocampal cultures and may contribute to the production and maintenance of SRSs in this model.

Localization of a r-protein gene within the chloroplast DNA replication origin of Chlamydomonas

In our previous study of chloroplast (Cp) DNA replication in Chlamydomonas reinhardtii, one D-loop site with its flanking regions was cloned and sequenced. The D-loop site mapped by electron microscopy (EM) overlaps with an open reading frame (ORF) potentially coding for a polypeptide of 136 amino acids. In this report, the corresponding D-loop isolated from another species of Chlamydomonas was sequenced. An ORF was also detected. Sequence comparison indicated that most conserved sequences between these two cloned origins are located within the ORF. Amino acid sequences of these two ORFs are highly conserved. The corresponding sequence for this ORF in the tobacco Cp genome was located by a Southern blotting analysis. Since the complete sequence data of Cp DNAs from a liverwort and from tobacco have been determined in 2 Japanese laboratories recently, it has been possible for us to show that this ORF encodes a protein homologous to the Cp ribosomal protein (r-protein) L16, by sequence comparison.