Meiotic Gene Conversion Mutants in SACCHAROMYCES CEREVISIAE . I. Isolation and Characterization of pms1-1 and pms1-2

The pms1 mutants, isolated on the basis of sharply elevated meiotic prototroph frequencies for two closely linked his4 alleles, display pleiotropic phenotypes in meiotic and mitotic cells. Two isolates carrying recessive mutations in PMS1 were characterized. They identify a function required to maintain low postmeiotic segregation (PMS) frequencies at many heterozygous sites. In addition, they are mitotic mutators. In mutant diploids, spore viability is reduced, and among survivors, gene conversion and postmeiotic segregation frequencies are increased, but reciprocal exchange frequencies are not affected. The conversion event pattern is also dramatically changed in multiply marked regions in pms1 homozygotes. The PMS1 locus maps near MET4 on chromosome XIV. The PMS1 gene may identify an excision-resynthesis long patch mismatch correction function or a function that facilitates correction tract elongation. The PMS1 gene product may also play an important role in spontaneous mitotic mutation avoidance and correction of mismatches in heteroduplex DNA formed during spontaneous and UV-induced mitotic recombination. Based on meiotic recombination models emphasizing mismatch correction in heteroduplex DNA intermediates, this interpretation is favored, but alternative interpretations involving longer recombination intermediates in the mutants are also considered.     

Biosynthesis of the beta-lactam antibiotic, thienamycin, by Streptomyces cattleya

Radioactive- and stable isotope-containing substrates were used to identify the biosynthetic precursors of the beta-lactam antibiotic, thienamycin, in Streptomyces cattleya. Acetate is utilized by the organism to form C(6) and C(7) of the beta-lactam ring. The two carbons of the hydroxyethyl group attached to C(6) are both derived from the methyl of methionine. The cysteaminyl side chain attached to C(2) is derived from cysteine. Selective inhibition of thienamycin and cephamycin C biosynthesis has been achieved either through the addition of metabolic inhibitors or through manipulation of the growth medium. These results suggest that the two beta-lactam antibiotics, thienamycin and cephamycin C, are formed by different biosynthetic pathways.     

Ram-induced growth of ovarian follicles and gonadotrophin inhibition in anoestrous ewes

Southdown ewes in mid-seasonal anoestrus were exposed to rams for 0 h (control group), 2 h, 24 h, 40 h, 3 days, 10 days or 20 days. Serial blood samples were then taken to determine LH and FSH levels. Ewes with greater than 24 h ram exposure were ovariectomized immediately after bleeding, and all follicles greater than 1 mm diameter were dissected from the ovaries and measured. LH basal concentrations and pulse frequency increased significantly within 2 h of ram introduction, but by 24 h fell, and then remained low. FSH concentrations fell within 2 h of ram introduction and remained low. Control group ewes (isolated) had no follicles greater than 4 mm diameter, whereas all ewes exposed to rams had large follicles, with CL or preovulatory follicles present at 40 h after ram introduction. Ram introduction was also associated with follicle recruitment (antrum formation to less than 2 mm). Follicular recruitment and development to the large follicle stage therefore occurred during a period of low plasma gonadotrophin levels and suppressed LH pulsing.     

The structural gene for human coagulation factor X is located on chromosome 13q34

The gene coding for coagulation factor X was studied in a family segregating chromosomal abnormalities involving chromosomes 13 and 6. An individual monosomic for 13q34 was deficient in levels of clotting factors VII and X, while her brother, who is trisomic for 13q34, had elevated levels. DNA dosage studies with a cloned human factor X gene demonstrated that the low levels of factor X expression in the individual with the chromosome 13q34 deletion were due to the absence of one copy of the factor X structural gene. This confirms the assignment of the human gene coding for factor X to 13q34.     

Sugar,fat, and the risk of colorectal cancer.

The habitual diet of 50 patients with large bowel cancer, as assessed by a dietary history method, was compared with that of 50 closely matched controls. Patients were included only if their symptoms were unlikely to have changed previous eating habits. The mean daily intakes of all major nutrient classes and of dietary fibre were estimated. Patients with large bowel cancer consumed 16% more energy than controls (mean (SEM) daily intake 9.92 (0.41) v 8.56 (0.32) MJ (2370 (98) v 2046 (76) kcal), respectively; p less than 0.0001), mainly in the form of carbohydrate (21% more; 282.6 (13.7) v 233.4 (10.5) g; p less than 0.0001) and fat (14% more; 100.8 (4.3) v 88.4 (3.2) g; p less than 0.001). The extra carbohydrate was largely in the form of sugars depleted in fibre and the extra fat as combinations of fat and such sugars. As the selection criteria used make it unlikely that this eating pattern was caused by the disease the data suggest that a high intake of sugars depleted in fibre and fat predisposes to the development of large bowel cancer.     

Disruption of phospholipid asymmetry in erythrocyte vesicles deficient in spectrin

The transbilayer distribution of phospholipids in right-side-out and inside-out vesicles derived from human erythrocytes was studied by phospholipase A2 digestion assays and by staining with the fluorescent dye merocyanine 540. In both types of vesicles, the normal asymmetric distribution of phospholipids characteristic of intact cells was disrupted. Because both types of vesicles are deficient in spectrin, the major protein of the cytoskeletal network which normally underlies the membrane, these results support the contention that spectrin is involved in the maintenance of phospholipid asymmetry.     

Identification of a deletion in the low density lipoprotein (LDL) receptor gene in a patient with familial hypercholesterolaemia

Summary DNA samples from 60 unrelated UK patients with familial hypercholesterolaemia (FH) were screened by Southern blot hybridisation to detect gross alterations in the low density lipoprotein (LDL) receptor gene. One patient was found to have a 2kb deletion in the 3 part of the gene. The deletion cosegregates with the FH phenotype in his family. This finding is compatible with the deletion being the cause of FH in this case and makes a presymptomatic test based on DNA analysis available for this family. The defects in most of the other patients are likely to be due to point mutations.     

Temperature-Sensitive Lethal Mutations on Yeast Chromosome I Appear to Define Only a Small Number of Genes

A method was developed for isolating large numbers of mutations on chromosome I of the yeast Saccharomyces cerevisiae. A strain monosomic for chromosome I (i.e., haploid for chromosome I and diploid for all other chromosomes) was mutagenized with either ethyl methanesulfonate or N-methyl-N'-nitro-N -nitrosoguanidine and screened for temperature-sensitive (Ts^- ) mutants capable of growth on rich, glucose-containing medium at 25° but not at 37°. Recessive mutations induced on chromosome I are expressed, whereas those on the diploid chromosomes are usually not expressed because of the presence of wild-type alleles on the homologous chromosomes. Dominant ts mutations on all chromosomes should also be expressed, but these appeared rarely. — Of the 41 ts mutations analyzed, 32 mapped on chromosome I. These 32 mutations fell into only three complementation groups, which proved to be the previously described genes CDC15, CDC24 and PYK1 (or CDC19). We recovered 16 or 17 independent mutations in CDC15, 12 independent mutations in CDC24 and three independent mutations in PYK1. A fourth gene on chromosome I, MAK16, is known to be capable of giving rise to a ts-lethal allele, but we recovered no mutations in this gene. The remaining nine mutations isolated using the monosomic strain appeared not to map on chromosome I and were apparently expressed in the original mutants because they had become homozygous or hemizygous by mitotic recombination or chromosome loss. — The available information about the size of chromosome I suggests that it should contain approximately 60–100 genes. However, our isolation in the monosomic strain of multiple, independent alleles of just three genes suggests that only a small proportion of the genes on chromosome I is easily mutable to give a Ts^--lethal phenotype. — During these studies, we located CDC24 on chromosome I and determined that it is centromere distal to PYK1 on the left arm of the chromosome.