The surprising complexity of signal sequences

Most secreted and many membrane proteins contain cleavable N-terminal signal sequences that mediate their targeting to and translocation across the endoplasmic reticulum or bacterial cytoplasmic membrane. Recent studies have identified many exceptions to the widely held view that signal sequences are simple, degenerate and interchangeable. Growing evidence indicates that signal sequences contain information that specifies the choice of targeting pathway, the efficiency of translocation, the timing of cleavage and even postcleavage functions. As a consequence, signal sequences can have important roles in modulating protein biogenesis. Based on a synthesis of studies in numerous experimental systems, we propose that substrate-specific sequence elements embedded in a conserved domain structure impart unique and physiologically important functionality to signal sequences.     

Constitutive heterochromatin distribution in monocentric and polycentric chromosomes

Monocentric chromosomes of Vicia faba (2n=12) and polycentric chromosomes of Luzula purpurea (2n=6) and L. multiflora (2n=36) were studied by the C-banding technique. C-positive regions exhibited a restricted distribution in V. faba nuclei and were located near the centromeres of the chromosomes. Each chromosome had both a characteristic number and distribution of C-positive regions permitting homologue identification. L. purpurea and L. multiflora C-bands were much more numerous than those of V. faba and were widely distributed throughout both nuclei and chromosomes. Three distinct constitutive heterochromatin distribution patterns were present in L. purpurea metaphase chromosomes permitting homologous chromosome identification. One of three C-band distribution patterns was also evident in L. multiflora chromosomes.     

Constitutive heterochromatin and transposable elements in Drosophila melanogaster

Several families of transposable elements (TEs), most of them belonging to the retrotransposon catagory, are particularly enriched in Drosophila melanogaster constitutive heterochromatin. The enrichment of TE-homologous sequences into heterochromatin is not a peculiar feature of the Drosophila genome, but appears to be widespread among higher eukaryotes. The constitutive heterochromatin of D. melanogaster contains several genetically active domains; this raises the possibility that TE-homologous sequences inserted into functional heterochromatin compartments may be expressed. In this review, I present available data on the genetic and molecular organization of D. melanogaster constitutive heterochromatin and its relationship with transposable elements. The implications of these findings on the possible impact of heterochromatic TEs on the function and evolution of the host genome are also discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Constitutive heterochromatin patterns of G-group chromosomes in Down's syndrome

Summary 2 male patients with G-trisomic Down's syndrome, were studied for constitutive heterochromatin pattern by the technique of Arrighi and Hsu (1971). 2 G-chromosomes were found to have heterochromatin distributed around the centromeric areas and the remaining 3 were free of such localized heterochromatin blocks; the extra G-chromosome belonging to the non-localized heterochromatin members.

---

Zusammenfassung 2 männliche Patienten mit G-Trisomie (Down-Syndrom) sind im Hinblick auf das konstitutive Heterochromatinmuster mit Hilfe der Technik von Arrighi u. Hsu (1971) untersucht worden. Es fanden sich 2 G-Chromosomen, die eine Heterochromatinverteilung um die Zentromerregion zeigen, während die restlichen 3 frei von derartig lokalisierten Heterochromatinblocks sind; das überzählige G-Chromosom gehört zu den letztgenannten.

---

---

Supported in part by the Robert A. Welch Foundation, The National Foundation —March of Dimes and USPHS Grant No. FR-05425.     

Constitutive heterochromatin polymorphism and chromosome damage in viral hepatitis

The frequencies of chromosomal aberrations and sister-chromatid exchanges (SCEs) were scored in relation to constitutive heterochromatin in 100 patients with viral hepatitis B, 100 patients with viral hepatitis A and 100 age- and sex-matched normal controls. 23.4%, 15% and 4% of the cells showed chromosomal aberrations in patients with hepatitis B, hepatitis A and normal controls respectively. Non-random involvement of chromosomal aberrations were also noted in chromosome 1 of patients with hepatitis B and A as compared to normal controls. The frequencies of SCEs (mean +/- S.D.) were found to be 10.40 +/- 2.83 in hepatitis B and 8.70 +/- 2.34 in hepatitis A. These values were significantly higher than the SCE frequency (mean +/- S.D.) of 5.88 +/- 2.25 observed in normal controls (P less than 0.001). The intra-chromosomal distribution of SCEs revealed a relatively increased incidence of SCEs in chromosome 1 of patients with hepatitis B and A as compared to normal controls. Analysis of constitutive heterochromatin polymorphism showed chromosome 1 qh+ to be the most frequent variant in patients with hepatitis B and A as compared to normal controls. The increased involvement of C-band variant 1 qh+ in patients with hepatitis B and A as compared to normal controls may indicate that extra heterochromatin offers additional sites for viral integration.     

Constitutive heterochromatin in early embryogenesis of Drosophila melanogaster

Summary The formation of constitutive heterochromatin was studied during the embryonic development of Drosophila melanogaster, using the C-banding technique. During embryonic cleavage, C-banded material is not seen in mitotic chromosomes; the differentiation between euchromatin and heterochromatin only occurs at blastoderm. This event correlates with the establishment of position-effect variegation.     

AutoPrime: selecting primers for expressed sequences

A selection of evaluations from Faculty of 1000 covering microarrays to study antibiotic resistance, identifying SUMO substrate proteins, parallel SNP genotyping, intron origin and evolution and variation in CpG-island methylation in humans.     

Dendrite development: a surprising origin

Neurons extend elaborate dendrites studded with spines. Unexpectedly, this cellular sculpting is regulated by the origin recognition complex -- the core machinery for initiating DNA replication.     

Constitutive heterochromatin variation in two species of rattus with apparently similar karyotypes

Rattus blanfordi and R. cutchicus medius both have a chromosome complement of 2n=36 and all chromosomes except the submetacentric Y of R. blanfordi are acrocentric. The apparently similar karyotypes of the two species, however, show variations in the nature and quantity of C-band-positive constitutive heterochromatin (C-heterochromatin) as revealed by C- and G-banding and Hoechst 33258 fluorescence. R. blanfordi with large-sized X and Y chromosomes and conspicuously larger centromeric heterochromatin in all the autosomes as compared to that of R. cutchicus medius has much more C-heterochromatin in its genome than the latter. The variation in the quantity of C-heterochromatin has been accomplished without altering the morphology of the acrocentric chromosomes unlike other mammals in which variations have been reported to result generally in the addition or deletion of a totally heterochromatic second arm.     

A male-specific DNA probe detects heterochromatin sequences in a familial Yq- chromosome.

Using a recombinant DNA probe, we have demonstrated the presence of residual 3.4-kilobase (kb) repeat sequences in a family with a Yq- chromosome. The heterochromatin of this Y variant was not readily detectable with conventional chromosome-banding techniques. These data suggest that the breakpoint of the deletion occurs at the heterochromatin region proximal to the euchromatin/heterochromatin junction.