Source-sink relations in maize mutants with starch-deficient endosperms

Partitioning and translocation of photosynthates were compared between a nonmutant genotype (Oh 43) of corn (Zea mays L.) and two starch-deficient endosperm mutants, shruken-2 (sh2) and brittle-1 (bt1), with similar genetic backgrounds. Steady-state levels of ¹⁴CO₂ were supplied to source leaf blades for 2-hour periods, followed by separation and identification of ¹⁴C-assimilates in the leaf, kernel, and along the translocation path. An average of 14.1% of the total ¹⁴C assimilated was translocated to normal kernels, versus 0.9% in sh2 kernels and 2.6% in btl kernels. Over 98% of the kernel ¹⁴C was in free sugars, and further analysis of nonmutant kernels showed 46% of this label in glucose and fructose. Source leaves of mutant plants exported significantly less total photosynthate (24.0% and 36.3% in sh2 and bt1 compared to 48.0% in the normal plants) and accumulated greater portions of label in the insoluble (starch) fraction. Mutant plants also showed lower percentages of photosynthate in the leaf blade and sheath below the exposed blade area. The starch-deficient endosperm mutants influence the partitioning and translocation of photosynthates and provide a valuable tool for the study of source-sink relations.     

Sequencing of the 16S-23S spacer in a ribosomal RNA operon of Zea mays chloroplast DNA reveals two split tRNA genes

The nucleotide sequence of th 16S-23S spacer from a ribosomal RNA operon of Zea mays chloroplast DNA has been determined. It contains two tRNA genes, coding for tRNAlle (AUCU) and tRNAAla (GCGA), which are split by intervening sequences of 949 and 806 base pairs, respectively. Homology between the two introns suggests that they have a common origin.     

Alterations in plasma-membrane functions after poliovirus infection

After exposure of HeLa cells to poliovirus there is a rapid decline (within minutes) in fluorescence polarization of DPH (1,6-diphenyl-1,3,5-hexatriene). Within one hour after infection the (Na+/K+)ATPase activity of an isolated plasma-membrane-rich fraction is enhanced, the cell volume decreases, and the intracellular concentration of a potent low-molecular-weight inhibitor of host protein synthesis increases.     

Cellular uptake of L-lactate in mouse diaphragm.

Early uptake curves of L-lactate and of mannitol were measured in quartered, incubated mouse diaphragms. Uptake was determined at 15, 30, and 45 s for various concentrations of lactate in the external solution as well as in the presence and absence of the competitive inhibitor of lactate transport, alpha-cyano-4-hydroxycinnimate. In normal preparations, when the external lactate concentration was 10 mM or less, the ratio of lactate-to mannitol space in the tissue was 1.7. This value was nearly independent of time and of external concentration. In normal preparations, when the external lactate concentration was greater than 10 mM, the ratio of lactate-to-mannitol space rose with time. At a fixed time, however, this ratio fell with increasing lactate concentration. In the inhibited preparations, the ratio of lactate-to-mannitol space rose with time at all concentrations. When lactate concentration was greater than 5 mM, this ratio was independent of the external concentration. The results suggest that there are two modes of lactate entry into these muscle cells. Entry can occur by means of a saturable system. When external lactate concentration is low, entry rates for this process are rapid compared with diffusional rates. This system probably saturates at concentrations near 10 mM and can facilitate transport in either direction. In addition, an appreciable passive leak is present. This leak accounts for about one fourth of the membrane transfer when external lactate is low, but is equal to the carrier transfer when lactate concentration is 30 mM. A model was developed to describe the entry of a permeating solute, such as lactate, into an isolated tissue.     

The inefficiency of ribosomes functioning in Escherichia coli growing at moderate rates

It is generally agreed that ribosomes function with reduced efficiency (i.e. a smaller proportion is actually engaged in protein synthesis) in bacteria growing at low growth rates (doubling times greater than 2 h). This paper examines whether the efficiency is constant in bacteria growing at various rates corresponding to doubling times of less than 2 h. Because isotopic methods cannot be used in very rich media, turbidimetric methods have been extended to follow the kinetics of growth immediately following the shift-up of cultures of Escherichia coli ML308 growing in glucose minimal medium or succinate minimal medium into a very rich medium supporting a balanced doubling time of 17.4 min. It is concluded that the efficiency of ribosome participation in protein synthesis is higher in the very rich medium than in the two minimal media, which support doubling times of 43 and 65 min, respectively, at 37 degrees C.     

Acyldiglucosyldiacylglycerol-containing lipoteichoic acid with a poly(3-O-galabiosyl-2-O-galactosyl-sn-glycero-1-phosphate) chain from Streptococcus lactis Kiel 42172.

The lipoteichoic acid of Streptococcus lactis Kiel 42172 was isolated. The lipid portions were released by HF and were established to be 3-O-[O-alpha-D-galactopyranosyl-(1 leads to 6)-alpha-D-galactopyranosyl]-2-O-alpha-D-galactopyranosyl-sn-glycero-1-phosphate, they are joined by phosphodiester bonds nosyl)]glycerol. The repeating units of the hydrophilic chain were established to be 3-O-[O-alpha-D-galactopyranosyl-(1 leads to 6)-alpha-D-galactopyranosyl]-2-O-alpha-D-galactopyranosyl-sn-glycero-1-phosphate; they are joined by phosphodiester bonds at carbon atom 6 of the galabiosyl residues. The innermost unit is linked to the glycolipid by a phosphodiester presumably at C-6 of the outer glucosyl moiety. The hydrophilic chain is 7.4--11.8 units in length, measuring 12--19 nm is extended conformation. The content of 2.7--2.96 acyl groups per 2 glucosyl residues indicates that 70--96% of the glycolipid consists of acyldiglucosyldiacylglycerol. The novel poly(glycosylgly-cerophosphate) structure provided for the first time the oplipoteichoic acids are the sn-1 isomer which has previously been suggested from biosynthetic studies (Glaser, L., & Lindsay, B. (1974) Biochem. Biophys. Res. Commun. 59, 1131--1136).