The in vitro synthesis of extraneous RNA sequences by SP6 and T7 RNA polymerases from specific DNA templates is described. Transcription of templates prepared by digestion with restriction enzymes that leave 3' protruding ends resulted in the production of significant amounts of long, template-sized RNA transcripts which hybridized to vector DNA. Sequences copied from the noncoding template strand were among the extraneous transcripts. The presence of these sequences in probe preparations were detected in Southern and RNase protection hybridization assays. In contrast, transcription of DNA templates with blunt or 5' protruding ends yielded few RNA products as extraneous sequences.

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In Dictyostelium discoideum, the lysosomal enzyme a-mannosidase is first synthesized as an N-glycosylated precursor of Mr 140,000. After a 20-30-min lag period, up to 30% of the precursor molecules are rapidly secreted, whereas the rest remain cellular and are proteolytically processed (tt/2 = 8 min) to mature subunits of Mr 58,000 and 60,000. The secreted precursor is modified more extensively than the cellular form, as is revealed by differences in size, charge, and sensitivity to endoglycosidase H. Subcellular fractionation has shown that, following synthesis in the rough endoplasmic reticulum, the precursor is transported to a low density membrane fraction that contains Golgi membranes. Proteolytic processing takes place in these vesicles, since newly cleaved mature enzyme, but no precursor, co-fractionates with Iysosomes. Under conditions that disrupt vesicular membranes, the precursor remains associated with the membrane fraction, whereas the newly processed mature enzyme is soluble. Proteolytic cleavage of the precursor thus coincides with the release of the mature enzyme into the lumen of a lysosomal compartment. These findings suggest a possible mechanism for lysosomal targeting that involves the specific association of enzyme precursors with Golgi membranes.

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