Mini-F E protein: the carboxy-terminal end is essential for E gene repression and mini-F copy number control

Mini-F is a segment of the conjugative plasmid F consisting of two origins of replication flanked by regulatory regions, which ensure a normal control of replication and partitioning. Adjacent to the ori-2 origin is a complex coding region that consists of the E gene overlapped by three open reading frames with the coding potential for 9000 Mr polypeptides here designated 9 kd-1, 9 kd-2 and 9 kd-3. In this paper, we show that open reading frame 9 kd-3 is preceded by active promoter and Shine-Dalgarno sequences. The E coding region specifies: an initiator of replication, which acts at the ori-2 site; a function that negatively regulates the expression of the E gene; and a function involved in mini-F copy number control. To assign one of these functions to one of the overlapping coding sequence, we have isolated, characterized and sequenced mutations mapping in the E coding region. In this paper, we analyse two mutations (cop5 and pla25) that abolish the repression of the E gene. As these mutations affect the primary structure of protein E itself but not the 9 kd polypeptides, we conclude that protein E takes part in the negative regulation of its own synthesis. In addition, the localization of the cop5 and pla25 mutations indicates that the carboxy-terminal end of the E protein is involved in the autorepression function. The cop5 mutation causes an eightfold increase of the mini-F copy number. The pla25 mutation leads to the inability of the derived mini-F plasmid to give rise to plasmid-harbouring bacteria. The ways in which the cop5 and pla25 mutations may lead to such phenotypes are discussed in relation to the different functions mapping in the E coding sequence.     

Regulation of Gene Expression in Developing Zea mays Embryos: Protein Synthesis during Embryogenesis and Early Germination of Maize

Polypeptides synthesized in dissected embryos of Zea mays at different stages of embryogenesis and early germination have been characterized by their migration in two-dimensional gel electrophoresis. This analysis has been carried out with in vivo labeled polypeptides from excised embryos and with proteins synthesized in vitro in the rabbit reticulocyte system directed by poly(A⁺) RNA isolated from the different developmental stages. We have identified three main sets of expressed polypeptides: (a) embryonic set: this group of polypeptides is synthesized in young and mature embryos but not in early germination; (b) maturation set: this group of polypeptides is not present in young embryos and appears during the maturation period. Some of these polypeptides are still present in early germination while others disappear from stored mRNAs in dry embryos. One particular group from this set can be induced prematurely in young embryos by incubation with abscisic acid; and (c) germination set: this group of polypeptides is not expressed in the maturation period and appears after brief imbibition of the dry embryos.     

Reorganization of microtubules in endosperm cells and cell fragments of the higher plant Haemanthus in vivo

The reorganization of the microtubular meshwork was studied in intact Haemanthus endosperm cells and cell fragments (cytoplasts). This higher plant tissue is devoid of a known microtubule organizating organelle. Observations on living cells were correlated with microtubule arrangements visualized with the immunogold method. In small fragments, reorganization did not proceed. In medium and large sized fragments, microtubular converging centers formed first. Then these converging centers reorganized into either closed bushy microtubular spiral or chromosome-free cytoplasmic spindles/phragmoplasts. Therefore, the final shape of organized microtubular structures, including spindle shaped, was determined by the initial size of the cell fragments and could be achieved without chromosomes or centrioles. Converging centers elongate due to the formation of additional structures resembling microtubular fir trees. These structures were observed at the pole of the microtubular converging center in anucleate fragments, accessory phragmoplasts in nucleated cells, and in the polar region of the mitotic spindle during anaphase. Therefore, during anaphase pronounced assembly of new microtubules occurs at the polar region of acentriolar spindles. Moreover, statistical analysis demonstrated that during the first two-thirds of anaphase, when chromosomes move with an approximately constant speed, kinetochore fibers shorten, while the length of the kinetochore fiber complex remains constant due to the simultaneous elongation of their integral parts (microtubular fir trees). The half-spindle shortens only during the last one-third of anaphase. These data contradict the presently prevailing view that chromosome-to-pole movements in acentriolar spindles of higher plants are concurrent with the shortening of the half-spindle, the self-reorganizing property of higher plant microtubules (tubulin) in vivo. It may be specific for cells without centrosomes and may be superimposed also on other microtubule-related processes.     

Antigenic structure of histone H2B

Antigenic determinants of histone H2B were localized using a series of 23 overlapping fragments of H2B obtained either by chemical and enzymatic cleavage of the histone or by solid-phase peptide synthesis. The ability of peptides to bind H2B antibodies was measured in an enzyme-linked immunosorbent assay, using antisera directed against calf thymus and chicken erythrocyte H2B as well as four anti H2B monoclonal antibodies obtained from autoimmune mice. Seven antigenic determinants were localized in the H2B molecule in the vicinity of residues 1-11, 6-18, 15-25, 26-35, 50-65, 94-113 and 114-125. Two of these determinants (residues 6-18 and 26-35) were revealed only through the binding properties of antibodies isolated from autoimmune mice. The usual correlation between hydrophilicity and antigenicity was found to hold for four of the epitopes, and the N- and C-termini of H2B were both antigenically active.     

Specific recognition by the tripeptide lysyl-tryptophyl-alpha-lysine of structural damage induced in DNA by platinum derivatives

The antitumoral derivative cisPt binds to DNA, as do its inactive analogs, trans- and dienPt. Structural damage introduced into DNA after reaction with the Pt derivatives were probed by using the peptide LysTrpLys. This peptide was used for its preferential binding to single-stranded structures (Brun, F., Toulmé, J.J. and Hélène, C. (1975) Biochemistry 14, 558-563). Phosphorescence lifetime measurements show that the Pt-induced heavy atom effects are quite similar in the three peptide-DNA-Pt complexes whatever the nature of the Pt derivative used. In contrast, fluorescence quenching strongly depends on the nature of the Pt derivatives. This quenching was therefore attributed to the stacking interactions engaged by the tryptophan residue with nucleic acid bases. A comparison of fluorescence quenching data for native and modified DNAs demonstrates that modification by dienPt has no effect on stacking interactions and that high levels of modifications by trans Pt are required to observe a change in stacking efficiency. In contrast modification by cis Pt induces the formation of strong stacking sites. The results strongly suggest the existence of locally opened regions in DNA modified by cis Pt.     

Radiation-induced strand breaks in phi X174 replicative form DNA: an improved experimental and theoretical approach

To determine the yield of radiation-induced single-strand, double-strand and potential breaks (breaks which are converted into actual breaks by alkali or heat treatment) oxygenated aqueous solutions of phi X174 supercoiled circular double-stranded (RFI) DNA were irradiated with increasing doses of gamma-irradiation and subjected to electrophoresis on agarose gels both before and after heat treatment. A complete separation was obtained of RFI, RFII (relaxed circle due to one or more single-strand breaks) and RFIII (linear DNA due to one double-strand break). A computer-assisted spectrophotometric procedure was developed, which enabled us to measure very accurately the amount of DNA present in the three DNA fractions. The quantitative changes of each fraction of DNA with dose could be fitted to a straightforward statistical model, which described the dose-dependent formation of the different types of breaks and from which the D37-values of single-strand, potential single-strand and double-strand breaks could be calculated to be 0.42 +/- 0.02, 1.40 +/- 0.25 and 57 +/- 36 Gy respectively. Potential double-strand breaks were not formed significantly under our conditions. In addition the maximum distance between two independently introduced single-strand breaks in opposite strands resulting in a double-strand break could be determined. The values before and after heat treatment are shown to be 29 +/- 6 and 102 +/- 13 nucleotides, respectively.     

Oligonucleotides covalently linked to intercalating agents. Influence of positively charged substituents on binding to complementary sequences

A pentamethylene chain was used to covalently link the 3'-phosphate of oligothymidylates to the 9-amino group of an acridine derivative. Positively charged substituents were further attached to the 3'-phosphate group to form 3'-phosphotriesters. These molecules form specific complexes with poly(rA) which involve the formation of a number of A X T base pairs equal to that of thymines in the oligonucleotide. Absorption changes induced in the acridine absorption bands are similar to those expected upon intercalation of the acridine dye between A X T base pairs. The acridine covalently linked to the 3'-phosphate strongly stabilizes the complexes formed with poly(rA) as compared with the corresponding unsubstituted oligodeoxynucleotide. The presence of a positively charged substituent on the 3'-phosphate together with the acridine dye further enhances the interaction. The effect of salt concentration on complex stability depends on the number of negatively charged phosphate groups of the oligodeoxynucleotide and on the nature of the substituents borne by the 3'-phosphate group. When the oligothymidylate is substituted by an acridine dye, the stability of the poly(rA) complexes increases when salt concentration increases. If an additional positively charged substituent is present on the 3'-phosphate group, stability decreases when salt concentration increases for the shortest oligonucleotide (trimer) and increases with longer oligonucleotides. Thermodynamic parameters have been calculated from the concentration dependence of melting temperatures.     

The isolation and genetic analysis of aCaenorhabditis elegants translocation (szT1) strain bearing an X-chromosome balancer

A single X-chromosome balancer-bearingCelegans ♂+, as a founder of a strain (AF1), was isolated directly from Fl progeny of irradiated+dpy-8unc-3/lon-2++ hermaphrodites on the basis of the absence of recombinant F2 categories. The balancer chromosome (Bal-X-1) suppresses recombination over a two-thirds section of the X chromosome (between genesdpy-8 andlet-2) and is associated with a reciprocal translocation between linkage groups (LG) X and I. Animals homozygous for the translocation (szT1(X:1)) are nonviable. Hermaphrodites heterozygous for the translocation segregate male selfprogeny at a frequency of 0.08-0.12.Bal-X-l carries the marker mutationlon-2(e678) and can be detected cytologically. This balancer chromosome proved useful for rnaimaininga number of X-linked lethal mutations and deficiencies inC. elegans.