We isolated overlapping recombinant cosmids that represent the equivalent of two complete dihydrofolate reductase amplicon types from the methotrexate-resistant CHO cell line CHOC400. The type I amplicons are 260 kilobases long, are arranged in head-to-tail fashion, and represent 10 to 15% of the amplicons in the CHOC400 genome. The type II amplicons are 220 kilobases long, are arranged in head-to-head and tail-to-tail configurations, and constituted the majority of the remaining amplicons in CHOC400 cells. The type II amplicon sequences are represented entirely within the type I unit. These are the first compiete amplicons to be cloned from a mammalian cell line.

We have recently isolated overlapping recombinant cosmids that represent the equivalent of two complete dihydrofolate reductase (dhfr) amplicon types from the methotrexate-resistant Chinese hamster ovary (CHO) cell line CHOC 400. In the work described in this report, we used pulse-field gradient gel electrophoresis to analyze large SfiI restriction fragments arising from the amplified dhfr domains. The junction between the 260-kilobase type I amplicons (which are arranged in head-to-tail configurations in the genome) has been localized, allowing the construction of a linear map of the parental dhfr locus. We also show that the 220-kilobase type II amplicons are arranged as inverted repeat structures in the CHOC 400 genome and arose from the type I sequence relatively early in the amplification process. Our data indicate that there are a number of minor amplicon types in the CHOC 400 cell line that were not detected in previous studies; however, the type II amplicons represent ca. 75% of all the amplicons in the CHOC 400 genome. Both the type I and type II amplicons are shown to be composed entirely of sequences that were present in the parental dhfr locus. Studies of less resistant cell lines show that initial amplicons can be larger than those observed in CHOC 400. Once established, a given amplicon type appears to be relatively stable throughout subsequent amplification steps. We also present a modification of an in-gel renaturation method that gives a relatively complete picture of the size and variability of amplicons in the genome.

Gyrate atrophy of the choroid and retina (GA) is an autosomal recessive chorioretinal degeneration caused by deficiency of the mitochondrial matrix enzyme, ornithine-5-aminotransferase (OAT). To study the molecular basis of the mutations causing GA, we cloned and sequenced the human OAT cDNA and determined the intron-exon arrangement of the structural gene. Using the cDNA template, we synthesized antisense RNA probes and performed RNase A protection experiments with RNA from four Lebanese GA patients. We found a probe-target mismatch at the 5' end of the first coding exon and amplified this region of the patients' genomic DNA using the polymerase chain reaction. Sequence analysis showed a G A transition, changing the initiator ATG (methionine) codon to ATA. This mutation segregates with the GA allele in both pedigrees. Initiation of translation at the closest in-frame methionine codon would truncate OAT by 138 amino acids, eliminating the entire mitochondrial leader sequence and 113 amino acids of the mature peptide.

We have previously cloned and characterized two different dihydrofolate reductase amplicon types from a methotrexate-resistant Chinese hamster ovary cell line (CHOC 400

Gyrate atrophy of the choroid and retina (GA) is an inherited chorioretinal degeneration caused by deficiency of ornithine 8-aminotransferase (OAT; L-ornithine: 2-oxo-acid aminotransferase; EC 2.6.1.13). GA is one of the "Finnish genetic diseases," a group of several rare monogenic disorders that occur with increased frequency in the Finnish population. Using a combination of RNase A protection, genomic cloning, and polymerase chain reaction amplification of genomic DNA, we found one of two missense mutant OAT alleles to be present in each of 16 Finnish GA pedigrees. The first mutation R180T, in which arginine-180 is replaced by threonine, was present in homozygous form in patients from two pedigrees. The second mutation L402P, in which leucine-402 is replaced by proline, was present in homozygous form in patients from 14 pedigrees. Neither mutation was present in 19 Finnish controls. L402P was not present in 18 non-Finnish GA patients but R180T was found in an American GA patient. We constructed full-length mutant cDNAs by amplifying patient cDNA with the polymerase chain reaction and cloning a restriction fragment containing the mutation into an otherwise normal human OAT cDNA. These mutant cDNAs were then expressed in CHO-K1 cells, which lack endogenous OAT. Both R180T and L402P inactivate OAT. These results show molecular heterogeneity in GA alleles even in the Finnish population.Ornithine 8-aminotransferase (OAT; L-ornithine:2-oxo-acid aminotransferase; EC 2.6.1.13) is a mitochondrial matrix enzyme that catalyzes the reversible transamination of ornithine to Al-pyrroline 5-carboxylate (1). Deficiency of OAT in man causes the autosomal recessive chorioretinal degeneration, gyrate atrophy of the choroid and retina (GA). To study the cell biology of OAT and the mutations causing GA, we have cloned and sequenced a near full-length human liver OAT cDNA and have determined the structure of the human OAT structural gene, which has 11 exons and spans 22 kilobases (kb) (2). We (2, 3) and others (4, 5) have shown that the OAT structural gene is located on chromosome 10 and that there is a group of nonfunctional OAT-related sequences on the X chromosome.Three missense mutations of OAT causing GA have been reported. The first mutation, in which the initiation methionine was replaced with an isoleucine, was found in affected members of two unrelated pedigrees of Lebanese Maronite descent (6). GA probands from 17 pedigrees of a variety of other ethnic backgrounds, including Finnish, were shown to lack this mutation (6). The second mutation was the Val-332 -* Met change found in a pyridoxine-responsive patient, and the third was the Asn-54 --Lys change (7). The ethnic background of the latter two patients was not provided.The Finnish population, which expanded from -250,000 in 1700 A.D. to -5 million at present, has been isolated genetically by geographical, linguistic, and cultural barriers (8). For these reasons, the variety and frequency of monogenic disorders in Finns is quite different from other European pop...

We have sequenced mouse tRNA genes from two recombinant X phage.

Peak eustatic sea level (ESL), or minimum ice volume, during the protracted marine isotope stage 11 (MIS11) interglacial at ;420 ka remains a matter of contention. A recent study of high-stand markers of MIS11 age from the tectonically stable southern coast of South Africa estimated a peak ESL of 13 m. The present study refines this estimate by taking into account both the uncertainty in the correction for glacial isostatic adjustment (GIA) and the geographic variability of sea level change following polar ice sheet collapse. In regard to the latter, the authors demonstrate, using gravitationally self-consistent numerical predictions of postglacial sea level change, that rapid melting from any of the three major polar ice sheets (West Antarctic, Greenland, or East Antarctic) will lead to a local sea level rise in southern South Africa that is 15%-20% higher than the eustatic sea level rise associated with the ice sheet collapse. Taking this amplification and a range of possible GIA corrections into account and assuming that the tectonic correction applied in the earlier study is correct, the authors revise downward the estimate of peak ESL during MIS11 to 8-11.5 m.

IVD i s a m i t o c h o n d r i a l f l a v o p r o t e i n , a tetramer o f 43 kDasubunits, which c a t a l y z e s t h e t h i r d r e a c t i o n i n l e u c i n e metabol i s m . H e r e d i t a r y d e f i c i e n c y o f IVO causes i s o v a l e r i c acidemia, one o f t h e more common i n h e r i t e d o r g a n i c acidemias, c h a r a c t e r i zed by r e c u r r e n t episodes o f vomiting, ketoacidosis, l e t h a r g y and sweaty f e e t odor. Extensive molecular heterogeneity, i n c l u d i n g a t l e a s t f i v e d i s t i n c t v a r i a n t a l l e l e s , has been shown on t h e b a s i s o f t h e presence o r absence o f immunoprecipitable v a r i a n t IVDs and t h e i r s i z e s (PNAS 82:7081, 1985). As a f i r s t s t e p i n t h e study o f t h e molecular b a s i s o f i s o v a l e r i c acidemia, we have i s o l a t e d a cDNA clone encoding r a t IVD. F i r s t , we h i g h l y p u r if i e d t h e mRNA encoding IVD from r a t l i v e r by polysome immunop u r i f i c a t i o n u s i n g p o l y c l o n a l , monospecific antibody. A cDNA l i b r a r y , enriched f o r IVD, was prepared u s i n g t h e p u r i f i e d mRNA, and screened by c o l o n l y h y b r i d i z a t i o n u s i n g two o l i g o n u c l e o t i d e s (17-mer and 15-mer) corresponding t o t h e p o r t i o n s o f t h e NHt e r m i n a l amino a c i d sequence.Numerous p o s i t i v e cDNA c l on& were i s o l a t e d . One o f them contained a 54 bp p o r t i o n which perf e c t l y matched an 18 amino a c i d p e p t i d e from t h e amino terminus o f pure r a t IVO, thereby p o s i t i v e l y i d e n t i f y i n g t h e c l o n e as t h a t o f r a t IVD. T h i s cDNA extends 100 bp towards t h e 5 ' end, i n d i c a t i n g t h a t i t c o n t a i n s t h e e n t i r e sequence f o r t h e l e a d e r p e p t i d e ( 2 kDa). Screening o f human cDNA l i b r a r i e s f o r IVD cDNA r c i n n m n m c c Second trimester MSAFP screening for low values is proving valuable in the detection of fetal chromosome defects in women 4 3 5 yrs. of age. Detection prior to 16 wks. would be desirable. To determine whether low MSAFP values also occur in the first trimester in association with a fetal chromosomal abnormality, we used a "simultaneous-sandwich" radioimmunoassay with polystyrene beads coated with anti-AFP monoclonal antibodies, using solid phase support. The coefficient of variation was45% for this assay which is about 10 x more sensitive than conventional RIAs .Blood was sampled in 359 cases just before CVS (chorionic villus sampling) in Milan. Fetal age was usually assessed by 2 ultrasound studies' 1 wk. apart, and chromosomes of the villi analysed. Sera were sent blind & only after AFP assay results were sent to Milan were karyotypes released. MSAFPL0. Univ o f Toronto, Ontario. MCAD i s a mitochondria1 f l a v o p r o t e i n , a tetramer o f 45 kDa OAT i s a homotetrameric, mitochondria1 m a t r i x enzyme d e f isubunits, which c a t a l y z e s t h e f i r s t r e a c t i o n i n t h e b -o x i d a t...