Amplification of the DFR1 gene in Saccaromyces cerevisiae

A new system was employed to study amplification of t,he DHF'R gene DFB,1 ) in Sa<,:;charoillYCB§. .Q~~Yi...S!i<;1~. . This system consists of a series of yeast strains containing a casset,te which encodes t he yeast, D..ERl gene ttghtly linked tjO a f usion of the yeast 1EU2. regulat,ory region wi tJ1 the LAQZ str ctural gene from E. cO.1-1 (,) . M. Clement , unpubl i,::;hed) . Th's casset;t e was shown t.o be integrat,ed int o a unj que chromosomal l ocati on in each strain . Yeast cells were se l ected for MTX-resistance and overproduction of ~ galac t osi d se ( B-gal ). Since the inserted DF'Rl and ~ACZ genes are independently regulated, it was thought that cel l s with this phenotype probably contain e d ampl if ications of the cassette. A lar ge variat ion in the f requn y o f MTX-resistance was found between the di ff e r ent str ains. These freqlen c ~ es r anged from about 2 x 10 - 7 fo r a population of cells containing the cassette integrated at, the BI J2.l gene in t,he middle of the long arm of chromosome V, to about 5 x 10-4 for a strain with the cassette i nserted in the r DNA cluster Abo It 85% of the MTX- res i stcmt iso l ates examined showed enhanced B·-gal act i v ity rel a t ive t o the parental strain . For the ma jorit y of strains, the mean B- gal activity in drug-r sistant clones was about 3 times that o f the parent following a single se l ect i on step . I n con t r ast, primary MTX-resistant derivat~ves of cells with the cassette inserted 3 at the rDNA cluster showed inc r eases in B- gal activity ranging from 9 - 14 f old r elative to the parent. Analysis of the latte r s train by Southe rn hybr idization indicated that the cassette was inde e d amplified several fold in MTX-re sistant derivatives. A sing l e strain, in which the cassette was inserted at the !lEA;], loc u.s , was used to examine in more detai 1 , the parameters affecting DFRl gene amplificat~ion in yeast . The mean B- gal activity in drug-resistant derivatives of this strain could be increased from 3 to 6 or 7 fold relative to the parent, by stepwise sel ection using increasing MTX concentrations. B-gal overproduction was found to be un stable in all primary and highly -resistant isolates examined. There was no indication, h owever, of a decrease i n growth r a t e in MTX-res i s tant cells which overproduced B - gal.

Visual word learning in good and poor readers

Forty students from regular, grade five classes were divided into two groups of twenty, a good reader group and a' poor reader group, on the basis. of their reading scores on Canadian Achievement Tests. .The subjects took. part in four experimental conditions iM which they .learned lists of pronounceable and unprono~nceable pseudowords, some with semantic referents, and responded to questions designed tci test visual perceptu~l learning and lexical ·and semantic association learning. It' was hypothesized "that the good reade~ group would be able to make use of graphemic and phonemic redundancy patterns in order to improv~·visuSl perceptual learning and lexical and semantic association lea~ningto a greater extent. than would .the poor reader gr6up. The data supported this hypothesis, and also indicated that, although the poor readers were less adept at using familiar sound and letter patterns, they were more dependent on· such pa~terns as an aid to visual recognition memory and semantic recall than were the good readers. It wa.s postulated that poor readers are in a double- ~ . bind situatio~ of having to choose between using weak graphemic-semantic associations or gr~pheme-phoneme associations which are also weak and which have hindered them in developing automaticity in. reading.