DO PRE-mRNAS PRESENT SECONDARY STRUCTURE IN VIVO?
Two properties of RNA molecules cannot be denied: their natural tendency to form highly stable secondary and tertiary structures in vitro and in vivo (9, 27, 39) and the observation that alterations in these structures represent a well-known regulatory mechanism for many RNA cellular processes (60). In this particular respect, however, a question that still remains to be addressed conclusively regards the presence of secondary structures in pre-mRNAs in vivo. That this existence may not simply be taken for granted comes from early experimental evidence. In fact, it was suggested that in vitro evidence regarding the possible influence of RNA structure on splicing (94) could not be accurately reproduced in vivo (95). The reason why this should be so goes back to the classical concept that RNA is coated in vivo by proteins. In fact, heterogeneous ribonucleoprotein particles have been known since early studies and the major protein family involved, the hnRNP proteins, are very abundant in
