Different subfamilies of alphoid repetitive DNA are present on the human and chimpanzee homologous chromosomes 21 and 22.

The alphoid repeat DNA on chimpanzee chromosome 22 was compared with alphoid repeat DNA on its human homologue, chromosome 21. Hybridization of different alphoid probes under various conditions of stringency show that the alphoid repeats of chimpanzee chromosome 22 are not closely related to those of human chromosome 21. Sequence analysis of cloned dimer and tetramer EcoRI fragments from chimpanzee chromosome 22 confirm the low overall level of homology, but reveal the presence of several nucleotide changes which are exclusive to the chromosome 21 subfamily of human alphoid DNA. Southern blot analysis of alphoid repeat DNA on the chimpanzee X chromosome suggests this subfamily has been strongly conserved during and since the separation of chimpanzee and man although the two subfamilies can be distinguished on the basis of Taq I restriction fragments.     

Import of an incompletely folded precursor protein into isolated mitochondria requires an energized inner membrane, but no added ATP.

We have studied the post-translational import of incomplete precursor chains into isolated yeast mitochondria. The precursor was a fusion protein containing a mitochondrial presequence attached to mouse dihydrofolate reductase. In vitro-synthesis of the precursor was interrupted by the elongation inhibitor cycloheximide and the arrested nascent chains cosedimenting with ribosomes were released by EDTA. These incomplete chains were efficiently imported by isolated yeast mitochondria; their import resembled that of the complete precursor in requiring an energized inner membrane and a mitochondrial presequence. It differed from that of the completed precursor in its resistance to methotrexate (which only binds to correctly folded dihydrofolate reductase) and its independence of added ATP. The incomplete chains were also more sensitive to proteinase K than the completed precursor. We conclude that the incomplete chains were incompletely folded and suggest that the lack of tight folding caused import into mitochondria to become independent of added ATP. This implies that ATP may participate, directly or indirectly, in the unfolding of the precursor for its transport into mitochondria.     

Calcium regulates inositol 1,3,4,5-tetrakisphosphate production in lysed thymocytes and in intact cells stimulated with concanavalin A.

Lysed mouse thymocytes release [3H]inositol 1,4,5 trisphosphate from [3H]inositol-labelled phosphatidyl inositol 4,5-bisphosphate in response to GTP gamma S, and rapidly phosphorylate [3H]inositol 1,4,5-trisphosphate to [3H]inositol 1,3,4,5-tetrakisphosphate. The rate of phosphorylation is increased approximately 7-fold when the free [Ca2+] in the lysate is increased from 0.1 to 1 microM, the range in which the cytosolic free [Ca2+] increases in intact thymocytes in response to the mitogen concanavalin A. Stimulation of the intact cells with concanavalin A also results in a rapid and sustained increase in the amount of inositol 1,3,4,5-tetrakisphosphate, and a much smaller transient increase in 1,4,5-trisphosphate. Lowering [Ca2+] in the medium from 0.4 mM to 0.1 microM before addition of concanavalin A reduces accumulation of inositol 1,3,4,5-tetrakisphosphate by at least 3-fold whereas the increase in inositol 1,4,5-trisphosphate is sustained rather than transient. The data imply that in normal medium the activity of the inositol 1,4,5-trisphosphate kinase increases substantially in response to the rise in cytosolic free [Ca2+] generated by concanavalin A, accounting for both the transient accumulation of inositol 1,4,5-trisphosphate and the sustained high levels of inositol 1,3,4,5-tetrakisphosphate. Inositol 1,3,4,5-tetrakisphosphate is a strong candidate for the second messenger for Ca2+ entry across the plasma membrane. This would imply that the inositol polyphosphates regulate both Ca2+ entry and intracellular Ca2+ release, with feedback control of the inositol polyphosphate levels by Ca2+.     

Release of a chimeric protein into the medium from Escherichia coli using the C-terminal secretion signal of haemolysin.

Recently, we have identified a novel topogenic sequence at the C terminus of Escherichia coli haemolysin (HlyA) which is essential for its efficient secretion into the medium. This discovery has introduced the possibility of using this secretion system for the release of chimeric proteins from E. coli directly into the medium. We have now successfully fused this C-terminal signal to a hybrid protein containing a few residues of beta-galactosidase and the majority of the E. coli outer membrane porin OmpF lacking its own N-terminal signal sequence. We find that this chimeric protein is specifically translocated across the inner and outer membranes and is released into the medium. In addition, we have further localized the HlyA secretion signal to the final 113 amino acids of the C terminus. In fact, a specific secretion signal appears to reside at least in part within the last 27 amino acids of HlyA.     

Fluoride complexes of aluminium or beryllium act on G-proteins as reversibly bound analogues of the gamma phosphate of GTP.

Fluoride activation of G proteins requires the presence of aluminium or beryllium and it has been suggested that AIF4- acts as an analogue of the gamma-phosphate of GTP in the nucleotide site. We have investigated the action of AIF4- or of BeF3- on transducin (T), the G protein of the retinal rods, either indirectly through the activation of cGMP phosphodiesterase, or more directly through their effects on the conformation of transducin itself. In the presence of AIF4- or BeF3-, purified T alpha subunit of transducin activates purified cyclic GMP phosphodiesterase (PDE) in the absence of photoactivated rhodopsin. Activation is totally reversed by elution of fluoride or partially reversed by addition of excess T beta gamma. Activation requires that GDP or a suitable analogue be bound to T alpha: T alpha-GDP and T alpha-GDP alpha S are activable by fluorides, but not T alpha-GDP beta S, nor T alpha that has released its nucleotide upon binding to photoexcited rhodopsin. Analysis of previous works on other G proteins and with other nucleotide analogues confirm that in all cases fluoride activation requires that a GDP unsubstituted at its beta phosphate be bound in T alpha. By contrast with alumino-fluoride complexes, which can adopt various coordination geometries, all beryllium fluoride complexes are tetracoordinated, with a Be-F bond length of 1.55 A, and strictly isomorphous to a phosphate group. Our study confirms that fluoride activation of transducin results from a reversible binding of the metal-fluoride complex in the nucleotide site of T alpha, next to the beta phosphate of GDP, as an analogue of the gamma phosphate.(ABSTRACT TRUNCATED AT 250 WORDS)     

The accumulation of mature RNA for the Xenopus laevis ribosomal protein L1 is controlled at the level of splicing and turnover of the precursor RNA.

A specific control regulates, at the level of RNA splicing, the expression of the L1 ribosomal protein gene in Xenopus laevis. Under particular conditions, which can be summarized as an excess of free L1 protein, a precursor RNA which still contains two of the nine introns of the L1 gene accumulates. In addition to the splicing block the two intron regions undergo specific endonucleolytic cleavages which produce abortive truncated molecules. The accumulation of mature L1 RNA therefore results from the regulation of the nuclear stability of its precursor RNA. We propose that a block to splicing can permit the attack of specific intron regions by nucleases which destabilize the pre-mRNA in the nucleus. Therefore the efficiency of splicing could indirectly control the stability of the pre-mRNA.     

(dC-dA)n.(dG-dT)n sequences have evolutionarily conserved chromosomal locations in Drosophila with implications for roles in chromosome structure and function.

In situ hybridization of (dC-dA)n.(dG-dT)n to the polytene chromosomes of Drosophila melanogaster reveals a clearly non-random distribution of chromosomal sites for this sequence. Sites are distributed over most euchromatic regions but the density of sites along the X chromosome is significantly higher than the density over the autosomes. All autosomes show approximately equal levels of hybridization except chromosome 4 which has no detectable stretches of (dC-dA)n.(dG-dT)n. Another striking feature is the lack of hybridization of the beta-heterochromatin of the chromocenter. The specific sites are conserved between different strains of D. melanogaster. The same overall chromosomal pattern of hybridization is seen for the other Drosophila species studied, including D. simulans, a sibling species with a much lower content of middle repetitive DNA, and D. virilis, a distantly related species. The evolutionary conservation of the distribution of (dC-dA)n.(dG-dT)n suggests that these sequences are of functional importance. The distribution patterns seen for D. pseudoobscura and D. miranda raise interesting speculations about function. In these species a chromosome equivalent to an autosomal arm of D. melanogaster has been translocated onto the X chromosome and acquired dosage compensation. In each species the new arm of the X also has a higher density of (dC-dA)n.(dG-dT)n similar to that seen on other X chromosomes. In addition to correlations with dosage compensation, the depletion of (dC-dA)n.(dG-dT)n in beta-heterochromatin and chromosome 4 may also be related to the fact that these regions do not normally undergo meiotic recombination.     

Extracellular release of colicin A is non-specific.

The possible involvement of topogenic export sequences within the colicin A polypeptide chain has been investigated. Different constructs have been made using various techniques to introduce deletions in the central and NH2-terminal regions of colicin A. Together, these deletions span the region from amino acid 15 to the end of the protein. None of these regions was found to be required for extracellular release or had any effect on the efficiency of this process. By inserting a termination codon, a Shine-Dalgarno sequence and an initiation codon into the gene for colicin A, the NH2-terminal and central plus COOH-terminal domains could be demonstrated to be released to the same extent when produced as separate polypeptides as when produced as linked ones. The introduction into the COOH-terminal domain of mutations promoting cytoplasmic aggregation had no effect on the secretion of the NH2-terminal polypeptide. These results demonstrated that no specific interaction between the NH2- and COOH-terminal regions of the colicin A polypeptide chain is involved in the release of colicin A. We are led to conclude that there is no topogenic export signal in the polypeptide chain of colicin A involved in the release mechanism. Thus the process is non-specific with respect to the colicin itself and depends solely on the expression of the colicin A lysis protein (Cavard et al., 1985, 1987). The expression of the protein causes the release of not only the colicin but also many other cellular proteins, including beta-lactamase, EF-Tu, and chloramphenicol acetyltransferase.