MESSENGER-RNA STRUCTURE, INSITU, AS ASSESSED BY MICROSCOPIC TECHNIQUES

The secondary and tertiary structure of RNA, in situ, is thought to be involved in distinct functions such as directing association of the RNA with the cytoskeleton, enzymatic activity of some RNAs, and the control of translation. In situ transcription (IST), a procedure by which cDNA is synthesized in situ, has been used to assess mRNA structure in situ using fixed cells or tissues. Distinct banding patterns were noted for mouse and rat POMC. Unique IST banding patterns were observed when an oligonucleotide complementary to a putative POMC stem-loop structure was used to prime IST. Indeed local changes in banding patterns could be elicited by pharmacological agents which modulate POMC translation. Inhibition of POMC synthesis with NaF or dexamethasone decreased the number of POMC mRNAs in the polysome fractions and increased the intensity of high molecular weight IST-derived bands. Forskolin, a stimulator of POMC synthesis, had the opposite effect. One mechanism by which translational control is thought to occur is by regulation of ribosome movement down the mRNA by specific binding of cytosolic proteins to RNA structure. Cytosolic protein fractions from AtT20 pituitary cells have been shown to specifically bind to the IST-predicted RNA structure. These findings suggest that 1) mRNA structure can be assessed in situ, 2) translation may be altered by the secondary and tertiary structure of mRNAs, and 3) a predicted stem-loop structure exists in situ in the 5'-end of POMC mRNA.