Transport of Vitamin B12 in Escherichia coli: Genetic Studies

The chromosomal location of two genetic loci involved in the transport of cyanocobalamin (B(12)) in Escherichia coli K-12 was determined. One gene, btuA, is believed to code for the transport protein in the cytoplasmic membrane, because a mutant with an alteration in this gene has lost the ability to accumulate B(12) within the cell although normal levels of the surface receptors for B(12) are present. The other locus, btuB, apparently codes for the surface receptor on the outer membrane. These mutants have lost the ability to bind B(12) and have greatly reduced transport activity, although growth experiments have shown that they can utilize B(12) for growth, but with decreased efficiency. This surface receptor for B(12) also appears to function as the receptor for the E colicins, because btuB mutants are resistant to the E colicins, and mutants selected for resistance to colicin E1 are defective in B(12) binding and transport. The gene order was determined by transduction analysis to be cyc-argH-btuA-btuB-rif-purD. In addition, mutations in metH, the gene for the B(12)-dependent homocysteine methylating enzyme, were obtained in this study. This gene was localized between metA and malB.     

Osmoregulation of the cane toad, Bufo marinus, in salt water

Adult cane toads, B. marinus, survived in salinities up to 40% sea-water (SW). Pre-exposure to 30, then 40% SW, increased the survival time of toads in 50% SW. Plasma from toads acclimated to salt water is hyperosmotic to the environment--a result of increased plasma sodium, chloride and urea concentrations. When toads were placed in tap-water and 20% SW, all significant changes to plasma sodium, chloride, urea and osmotic pressure occurred within the first 2 days of exposure. When toads were placed in 30 and 40% SW environments, the increases in plasma sodium and chloride concentrations occurred within the first 2 days of exposure while urea and total osmotic pressure continued to rise until some time between 2 and 7 days exposure.     

The effect of labour on uterine blood flow in the pregnant ewe.

The effect of labour on cardiac output and uterine blood flow was measured in pregnant ewes at a mean gestation of 124 days using radioactive microspheres labelled with 169Yb and 85Sr. Labour was induced by a continuous infusion of ACTH into the foetal circulation. Cardiac ouput measured before ACTH infusion in seven ewes was 5234 +/- 175-9 ml./min (mean +/- S.E.) and total uterine blood flow was 732 +/- 57-9 ml./min (mean +/- S.E.). Measurements during labour in six ewes showed a significant increase in cardiac output to 6175 +/- 149-6 ml./min (P less than 0-005) but no significant change in uterine blood flow. However, the partition of blood flow was altered; thus myometrial flow increased by 67% from 114 +/- 15-4 ml./min to 190 +/- 13-2 ml./min (P less than 0-005) while placental blood flow decreased, although not significantly, from 618 +/- 55-9 ml./min to 575 +/- 40-7 ml./min. Similar changes were observed in one ewe in spontaneous labour at term and in another ewe receiving an infusion of 4 mg oestradiol 17beta over a 24 hr period. It is concluded that labour is not associated with any major alternation in total uterine blood flow although myometrial blood flow is increased. It is not known whether this is due to the rise in circulating oestrogens which occurs prior to parturition in the ewe, or to other factors such as the work of uterine muscle during labour.