A UNIQUE U-5-]A SUBSTITUTION IN THE PHYSARUM-POLYCEPHALUM U1 SNRNA - EVIDENCE AT THE RNA AND GENE LEVELS

The 5' terminal sequence of U1 snRNA that base-pairs with the intron 5 ' splice site in the course of spliceosome assembly was considered to be universally conserved. A study of the P polycephalum U1 snRNA at bo th RNA and gene levels shows that there are exceptions to this rule: t he P polycephalum UI snRNA has a U to A substitution at position 5, th at is partially compensated by a high frequency of T residue at positi on +4 of introns. In contrast to the yeast genome, the P polycephalum genome contains several U1 snRNA coding sequences (about 20). They eit her encode the U1A snRNA expressed in microplasmodia or correspond to the previously cloned U1B coding sequence. Both coding sequences show the U(5)A substitution. The ratio of U1A ver-sus U1B coding sequences is of about 3. A U1A gene was cloned. The 60 nt region upstream of the coding sequence has the same sequence as in the U1B gene. The U1B gen e is probably expressed at another stage of the P polycephalum life cy cle.