Roles of Parental and Progeny DNA in Two Mechanisms of Phage S13 Recombination

THERE are at least two mechanisms for genetic recombination of phage S13–primary and secondary. They are distinguished by the amount of phage recombination observed in recombination-deficient (recA) bacterial hosts. An S13 cross performed in either of the recA hosts, Escherichia coli JC1553¹ or AB2463², yields a recombination frequency that is greatly reduced from that found in a rec⁺ host^{3, 4}; this suggests that the recA gene is required for the primary mechanism of S13 recombination. But even in a recA host the phage undergoes a small residual amount of recombination³ which has been attributed to a minor, secondary mechanism. Apparently the secondary mechanism functions in a recA cell and is only revealed when the primary mechanism is eliminated.     

Purine and Pyrimidine Synthesis by the Avian Malaria Parasite,Plasmodium lophurae*

SYNOPSIS. Purine and pyrimidine biosynthesis in the avian malaria parasite Plasmodium lophurae and its host cell, the duck erythrocyte, were investigated in vitro. Pyrimidine synthesis, as measured by the incorporation of C¹⁴-NaHCO₃ into cytosine, uracil and thymine was slight in uninfected duck erythrocytes, whereas infected erythrocytes and erythrocyte-free parasites had high rates of incorporation of NaHCO₃ into these bases. In addition, orotidine-5′-monophosphate pyrophosphorylase and thymidylate synthetase, 2 enzymes of the pyrimidine biosynthetic pathway, were found in cell-free extracts of the plasmodia. Purine synthesis was measured by determining the extent of incorporation of C¹⁴-Na-formate into adenine and guanine. Uninfected and infected erythrocytes had similar rates of Na-formate incroporation into adenine. whereas free parasites incorporated little of this compound into adenine, or guanine. On the other hand, the incorporation of Na-formate into guanine was 54% higher in infected erythrocytes than in uninfected erythrocytes. It is suggested that P. lophurae synthesizes purines to a limited extent, and derives most of its purines from the host erythrocyte. The greater incorporation of Na-formate into guanine by infected cells, and its low incorporation into free parasites may be accounted for by parasite conversion of host cell adenine (in the form of ATP) into guanine. Pyrimidine biosynthesis in infected cells can be accounted for by de novo synthesis by the parasite itself.     

The Effects of Agitated Liquid Medium on in vitro Cultures of Hevea brasiliensis

The initiation and prolonged growth of callus, from stem explants of young plants of Hevea brasilienies on solid medium yielded a heterogeneous callus, with areas which are the result of compact growth interspersed with brown necrotic tissue and soft white tissue formations. Subculturing this callus (O callus) to agitated liquid medium and returning it to solid medium resulted in the production of a homogeneous friable and rapidly growing callus (Rl callus) The two established lines O and Rl have remained stable over one year in culture and differ in gross morphology, anatomy, growth and auxin content. Both were maintained on Murashige and Skoog's medium, with 2 mg/1 2,4-D and 0.5 mg/I kinetin. R 1 but not O showed enhanced growth at the lower 2,4-D level of 0.2 mg/l: both lines failed to continue growing when 2,4-D was omitted. It is suggested that the changes resulting from subculture in agitated liquid medium are related to those undergone by callus cultures which become habituated. Thus the Rl callus line is regarded as partially habituated. Subculture in agitated liquid medium also resulted in the production of large numberr of polyploid cells but these did not persist over the long periods of subsequent growth on agar medium, Enhanced auxin production by the establihed Rl callus line was thus observed in the absence of a detectable level of polyploidy.     

Speculations on Structural Relationships between the Hypothalamic Releasing Factors of Pituitary Hormones

RECENTLY, hypothalamic releasing factors have been isolated from two different species (porcine and ovine) and their structures elucidated^1–5. These factors stimulate the secretion of pituitary hormones and have been shown to be small polypeptides. Thyrotropin releasing factor (TRF) for both species is the tripeptide pyroglutamyl-histidyl-proline amide (pGlu-His-Pro-amide)^1,2. TRF acts on pituitary thyrotrophs to stimulate the secretion of thyroid stimulating hormone (TSH). The structure of a hypothalamic factor which stimulates the secretion of the pituitary gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH) has been determined. This gonadotropin releasing factor, referred to as LRF, is a decapeptide and, like TRF, has both terminals blocked; in both species its primary sequence is pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-amide^3–5.     

Characterization of the Malaria Pigment (Hemozoin) from the Avian Malaria Parasite Plasmodium lophurae*

SYNOPSIS. Plasmodium lophurae hemozoin (malaria pigment) is a heme-containing protein which is distinctly different from hemoglobin and hematin by immunologic, spectrophotometric, fingerprint, heme-iron, gel filtration, and starch gel electrophoretic analyses. The calculated average molecular weight of P. lophurae hemozoin is ca. 40,000. Hemozoin contains at least 3 antigenic components and shows some indication of cross-reaction with hemoglobin.     

Mismatches between breeding success and habitat preferences in Hen Harriers Circus cyaneus breeding in forested landscapes

During the past century, the upland breeding areas of Hen Harriers in Ireland have been extensively afforested. There is no evidence that this species avoids breeding in heavily forested landscapes and, indeed, young commercial forests in their second rotation are often selected as nest‐sites. However, Hen Harriers have coexisted with these forested areas for only a few decades and it is possible that such landscapes are suboptimal. We examined the relationship between breeding success and habitat using a dataset spanning three years and four study areas in the south and west of Ireland. We assessed whether nest success and fledged brood size were related to habitat type, both at the nest‐site and in the surrounding landscape. Neither measure of breeding productivity was related to total forest cover or to percentage cover of closed canopy forest in the landscape. However, in a subset of areas, high cover of second‐rotation pre‐thicket (young forests planted on land from which a first rotation has already been harvested) in the surrounding landscape was associated with low levels of breeding success. This may be due to factors related to predation, disturbance or prey availability. The fact that second‐rotation pre‐thicket is a preferred habitat for nesting in Ireland suggests that Hen Harriers may be making suboptimal decisions in the landscapes available to them.