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Considerable progress has been made in the last decade in studies of the genetics of the inflammatory bowel diseases, Crohn's disease and ulcerative colitis. Epidemiological data, notably concordance rates in twin pairs and sibling pairs, have provided strong evidence for the importance of the genetic contribution, particularly in Crohn's disease. These observations provided the catalyst for laboratory-based studies of the molecular genetics of Crohn's disease and ulcerative colitis around the world. The complementary strategies of genome-wide scanning and candidate gene-directed studies have led to the identification of a number of genetic markers which appear to predict disease susceptibility and behaviour.

Novel Tn1/3 derivatives that contained either two left‐ or two right‐hand ends of the transposon were constructed in a small plasmid. Both transposed at reasonable frequencies to give normal transposition products, suggesting that only the 38‐bp inverted repeats of Tn3 are essential for transposition. Plasmids containing transposon derivatives with only one end (either left or right) undergo transposase‐dependent transposition between replicons at much lower frequencies, resulting in co‐integrate molecules in which there is no substantial duplication of transposon DNA and that appear to be simple fusions of the two plasmids. Both the right and left halves of the transposon are separately able to confer transposition immunity to the plasmid, this immunity being inseparably linked to transposition proficiency and specificity.

Novel Tn1/3 derivatives that contained either two left‐ or two right‐hand ends of the transposon were constructed in a small plasmid. Both transposed at reasonable frequencies to give normal transposition products, suggesting that only the 38‐bp inverted repeats of Tn3 are essential for transposition. Plasmids containing transposon derivatives with only one end (either left or right) undergo transposase‐dependent transposition between replicons at much lower frequencies, resulting in co‐integrate molecules in which there is no substantial duplication of transposon DNA and that appear to be simple fusions of the two plasmids. Both the right and left halves of the transposon are separately able to confer transposition immunity to the plasmid, this immunity being inseparably linked to transposition proficiency and specificity.

Transcriptional silencing is known to occur at centromeres, telomeres and the mating type region in the nucleus of fission yeast, Schizosaccharomyces pombe. Mating-type silencing factors have previously been shown also to affect transcriptional repression within centromeres and to some extent at telomeres. Mutations in the clr4+, rik1+ and swi6+ genes dramatically reduce silencing at certain centromeric regions and cause elevated chromosome loss rates. Recently, Swi6p was found to co-localise with the three silent chromosomal regions. Here the involvement of clr4+, rik1+ and swi6+ in centromere function is investigated in further detail. Fluorescence in situ hybridisation (FISH) was used to show that, as in swi6 mutant cells, centromeres lag on late anaphase spindles in clr4 and rik1 mutant cells. This phenotype is consistent with a role for these three gene products in fission yeast centromere function. The Swi6 protein was found to be delocalised from all three silent chromosomal regions, and dispersed within the nucleus, in both clr4 and rik1 mutant cells. The phenotypic similarity observed in all three mutants is consistent with the products of both the clr4+ and rik1+ genes being required to recruit Swi6p to the centromere and other silent regions. Mutations in clr4, rik1 and swi6 also result in elevated sensitivity to reagents which destabilise microtubules and show a synergistic interaction with a mutation in the beta-tubulin gene (nda3). These observations suggest that clr4+ and rik1+ must play a role in the assembly of Swi6p into a transcriptionally silent, inaccessible chromatin structure at fission yeast centromeres which is required to facilitate interactions with spindle microtubules and to ensure normal chromosome segregation.

The feasibility of using the fission yeast, Schizosaccharomyces pombe , as a host for the propagation of cloned large fragments of human DNA has been investigated. Two acentric vector arms were utilized; these carry autonomously replicating sequences ( ars elements), selectable markers ( ura4(+) or LEU2 ) and 250 bp of S. pombe terminal telomeric repeats. All cloning was performed between the unique sites in both vector arms for the restriction endonuclease Not I. Initially the system was tested by converting six previously characterized cosmids from human chromosome 11p13 into a form that could be propagated in S.pombe as linear episomal elements of 50-60 kb in length. In all transformants analysed these cosmids were maintained intact. To test if larger fragments of human DNA could also be propagated total human DNA was digested with Not I and size fractionated by pulsed field gel electrophoresis (PFGE). Fractions of 100-1000 kb were ligated to Not I-digested vector arms and transformed into S.pombe protoplasts in the presence of lipofectin. Prototrophic ura+leu+transformants were obtained which upon examination by PFGE were found to contain additional linear chromosomes migrating at between 100 and 500 kb with a copy number of 5-10 copies/cell. Hybridization analyses revealed that these additional bands contained human DNA. Fluorescent in situ hybridization (FISH) analyses of several independent clones indicated that the inserts were derived from single loci within the human genome. These analyses clearly demonstrate that it is possible to clone large fragments of heterologous DNA in fission yeast using this S.p ombe artificial chromosome system which we have called SPARC. This vector-host system will complement the various other systems for cloning large DNA fragments.

Using a cDNA probe for the gamma gene of protein kinase C (PKCG), an informative RFLP with a PIC value of 0.62 has been identified with the enzyme Mspl. The polymorphic bands have been assigned to chromosome 19. Analysis of the segregation of alleles for this probe in myotonic dystrophy families show several recombinants between PKCG and myotonic dystrophy (DM) and exclude this gene as a candidate for DM. Linkage relationships between PKCG and other loci on chromosome 19 are presented which exclude PKCG from the proximal region of chromosome 19 and which are consistent with the localization being at 19q13.2→qter.

Summary: The human gene locus c-MEL was identified following transfection of genomic DNA from the human melanoma cell line NK14; it has previously been assigned to chromosome 19 (p13.2–q13.2) by analysis of somatic cell hybrids. We have further refined the position of this gene to the proximal region of 19p (cen-p13.2), using cell hybrids containing only fragments of human chromosome 19. We have confirmed this physical localisation by linkage analysis with a recently described restriction fragment length polymorphism for the c-MEL gene, and mapped the locus within the region of the low density lipoprotein receptor gene (LDLR) (Lod 4.43, $$\hat \theta = 0.10$$ ) and the anonymous marker D19S11 (13.1.25) (Lod 9.33, $$\hat \theta = 0$$ ). This gene thus maps to a region of chromosome 19 involved in karyotypic abnormalities in a variety of malignancies including melanomas and leukaemias.

The two markers most closely linked to the myotonic dystrophy (DM) locus on chromosome 19 are the gene that codes for apolipoprotein CII (APOC2) and the anonymous probe D19S19 (LDR152). Both of these markers show tight linkage to DM, with maximum lod scores of>20 at recombination fractions of less than 0.05. We have identified, in a family in which DM segregates, an affected individual where a meiotic recombination event has occurred in which both of these linked markers have crossed over with the gene defect. This demonstrates that APOC2 and D19S19 are probably on the same side of DM.