The guanine-hypoxanthine permease GhxP of Erwinia amylovora facilitates the influx of the toxic guanine derivative 6-thioguanine.

AIM: Erwinia amylovora is the causal agent of fire blight, a devastating disease of apples and pears. This study determines whether the E. amylovora guanine-hypoxanthine transporter (EaGhxP) is required for virulence and if it can import the E. amylovora produced toxic analogue 6-thioguanine (6TG) into cells. METHODS AND RESULTS: Characterization of EaGhxP in guanine transport deficient Escherichia coli reveals that it can transport guanine, hypoxanthine and the toxic analogues 8-azaguanine (8AG) and 6TG. Similarly, EaGhxP transports 8AG and 6TG into E. amylovora cells. EaGhxP has a high affinity for 6TG with a Ki of 3·7 µmol l-1 . An E. amylovora ⊿ghxP::Camr strain shows resistance to growth on 8AG and 6TG. Although EaGhxP is expressed during active disease propagation, it is not necessary for virulence as determined on immature apple and pear assays. CONCLUSIONS: EaGhxP is not required for virulence, but it does import 6TG into E. amylovora cells. SIGNIFICANCE AND IMPACT OF THE STUDY: As part of the disease establishment process, E. amylovora synthesizes and exports a toxic guanine derivative 6TG. Our results are counter intuitive and show that EaGhxP, an influx transporter, can move 6TG into cells raising questions regarding the role of 6TG in disease establishment.

A comparison of Australian and Canadian calibration coefficients for air kerma and absorbed dose to water for 60Co gamma radiation.

Australian and Canadian calibration coefficients for air kerma and absorbed dose to water for 60Co gamma radiation have been compared using transfer standard ionization chambers of types NE 2561 and NE 2611A. Whilst the primary standards of air kerma are similar, both being thick-walled graphite cavity chambers but employing different methods to evaluate the Awall correction, the primary standards of absorbed dose to water are quite different. The Australian standard is based on measurements made with a graphite calorimeter, whereas the Canadian standard uses a sealed water calorimeter. The comparison result, expressed as a ratio of calibration coefficients R=N(ARPANSA)/N(NRC), is 1.0006 with a combined standard uncertainty of 0.35% for the air kerma standards and 1.0052 with a combined standard uncertainty of 0.47% for the absorbed dose to water standards. This demonstrates the agreement of the Australian and Canadian radiation dosimetry standards. The results are also consistent with independent comparisons of each laboratory with the BIPM reference standards. A 'trilateral' analysis confirms the present determination of the relationship between the standards, within the 0.09% random component of the combined standard uncertainty for the three comparisons.

The relative response of NE2561 and NE2611A ionization chambers in megavoltage x-ray beams.

The relative energy response of NE2561 and NE261 IA ionization chambers to megavoltage photon beams from the ARPANSA linac indicates significant differences between these two types of chamber. In 16 MV beams of TPR20(10) 0.779, differences of about 2% are observed. The results are expressed as ratios KQ of the beam quality correction factors kQ, where the kQ factor for each type of chamber is the ratio of the absorbed dose to water calibration factor ND, at the x-ray quality Q to that at 60Co. These results have implications for the use of generic kQ factors in dosimetry protocols and suggest that NE2561 and NE2611A ionization chambers cannot be assumed to be identical.

Standards and calibration facilities of the Australian Radiation Laboratory for ionizing radiation beams.

The Australian Radiation Laboratory (ARL) maintains national standards of exposure and working standards of absorbed dose. Some of the exposure standards maintained by ARL have been compared recently with those of the Bureau International des Poids et Mesures (BIPM) for 100 to 250 kV x-rays and 60Co gamma rays. The absorbed dose standard for 60Co gamma rays is compared regularly with other standards through the International Atomic Energy Agency (IAEA) and World Health Organization (WHO). All results show good agreement. A range of calibration qualities is offered at ARL, extending from 10 kV x-rays to 60Co gamma rays. A system of regional facilities has been developed for radiation protection calibrations; each facility is supplied with a set of 137Cs sources and an ionization chamber and electrometer for use with x-rays. The system is now almost complete throughout Australia.

Intercomparison of the national exposure standards of New Zealand and Australia.

An intercomparison of medium energy x-ray and 60Co gamma radiation exposure standards held by the Australian Radiation Laboratory and the National Radiation Laboratory (New Zealand) is reported. The standards agree within 0.4%.