Reduced translatable messenger RNA activities in leaves of barley infected with Erysiphe graminis f.sp. hordei

The translational activity of total RNA and polyadenylated (polyA⁺) RNA isolated from susceptible leaves of Hordeum vulgare infected with Erysiphe graminis f.sp. hordei has been investigated. When equal quantities of total RNA from control and inoculated leaves were included in assays of protein synthesis using rabbit reticulocyte lysates, the amount of protein synthesised in assays programmed with RNA from infected leaves was reduced to 61% of the controls at 1 day after inoculation. This effect increased with infection until at 5 days after inoculation, RNA from infected leaves had only 11% of the activity of that of controls. Analysis of translation products by SDS-polyacrylamide gel electrophoresis and fluorography indicated reduced synthesis of most labelled polypeptides in assays of RNA from infected leaves. Analysis of total RNA on agaroseformaldehyde gels indicated no significant degradation of differences in RNA from control and inoculated leaves. The proportion of total RNA present as polyA⁺ RNA also declined during infection but a decrease in the translational activity of equal amounts of polyA⁺ RNA from control and infected leaves was not observed until 5 days after inoculation. This decreased activity was due mainly to reduced synthesis of the M_r 20000 protein which was the major product in assays of polyA⁺ RNA from controls. The reduced activity of the polyA⁺ RNA at this later stage of infection was not evident in the presence of greater than 0·5 mm 7-methyl guanosine 5′ phosphate and the translation products were identical in the presence of this inhibitor. These data suggest that infection of barley by the powdery mildew fungus caused a rapid and extensive decline in the amount of available host mRNA.