The Kinetics of Selective Biological Transport: V. Further Data on the Erythrocyte-Monosaccharide Transport System

Measurements of the rate of loss of sugar from human erythrocytes into sugar-free solutions were made as a function of sugar concentrations. The half-saturation concentration of this process was found to be different from those half-saturation concentrations previously measured by other methods. These data, together with a number of similar data from former publications, are summarized in tabular form and their use in assessing postulated transport mechanisms is illustrated by consideration and rejection of a mechanism in which the transport process is assumed to be the result of a protein conformational change.     

Hydroxyproline arabinosides in the plant kingdom

The hydroxyproline-O-arabinosyl linkage is present in cell walls of selected tissues representing green plants from algae to angiosperms.     

The mechanism of sex determination in Rumex acetosella

Summary Cytogenetic studies were made with particular emphasis on the sex-determining mechanism in Rumex acetosella (6 x = 42) and its hybrids (F ₁, F ₂, BC ₁ and BC ₂) with R. hastatulus (synthetic 4 x = 16 = 4 A +4 X = and 4 x = 18 = 4 A + 2 (X Y ₁ Y ₂) = ). Rumex acetosella was almost strictly dioecious with 5050 male and female. Breeding tests revealed that the males were heterogametic. The longest chromosomes (S), usually two, are the sex chromosomes of this hexaploid species. The S chromosomes are homomorphic in both male and female. The sex chromosome: autosome ratios, and the strong epistatic male effect of the S ^M chromosome in the polyploid dioecious species and in the hybrids, are evidence of an X/Y Melandrium type sex-determining mechanism controlled by a single pair of homomorphic sex chromosomes. Thus, the sex chromosome formula of the males was S ^F S ^M and that of females was S ^F S ^F. The present approach is a new method for resolving the sex-determining mechanism in a dioecious species.     

Selection,linkage, and dominance in small populations

Summary Linkage, dominance, and selection interact significantly to alter the mean coefficient of inbreeding. The effect of one is not predictable without the other two. Close linkage between adjacent loci in the presence of intense selection caused a different response with overdominant gene action from with partial dominance. When selection was random, effects of linkage and dominance on the coefficient of inbreeding were nonexistent; but when selection was by either phenotype or genotype, linkage and dominance became important. Joint effects between linkage, dominance, and selection are illustrated in specific simulated populations.     

Book reviews

Ohne Zusammenfassung

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Book reviews

     

Peptidase mutants of Salmonella typhimurium

Six peptidase activities have been distinguished electrophoretically in cell extracts of Salmonella typhimurium with the aid of a histochemical stain. The activities can also be partially separated by chromatography on diethylaminoethyl-cellulose. These peptidases show overlapping substrate specificities. Mutants (pepN) of the parent strain leu-485 lacking one of these enzymes (peptidase N) were obtained by screening for colonies that do not hydrolyze the chromogenic substrate l-alanyl-beta-naphthylamide. The absence of this broad-specificity peptidase in leu-485 pepN(-) mutants allowed the selection of mutants unable to use l-leucyl-l-alaninamide as a leucine source. These mutants (leu-485 pepN(-)pepA(-)) lack a broad-specificity peptidase (peptidase A) similar to aminopeptidase I previously described in Escherichia coli. Mutants (pepD) lacking a dipeptidase (peptidase D) have been isolated from a leu-485 pepN(-)pepA(-) parent by penicillin selection for mutants unable to use l-leucyl-l-glycine as a leucine source. Mutants (pepB) lacking a fourth peptidase (peptidase B) have been isolated from a leu-485 pepN(-)pepA(-)pepD(-) strain by penicillin selection for failure to utilize l-leucyl-l-leucine as a source of leucine. Single recombinants were obtained by transduction for each of the peptidases missing in a leu-485 pepN(-)pepA(-)pepD(-)pepB(-) strain. The growth response of these recombinants to leucine peptides shows that all of these peptidases can function in the catabolism of peptides and that they display overlapping substrate specificities in vivo.