DISCORDANT EXPRESSION AND VARIABLE NUMBERS OF NEIGHBORING GGA-RICH AND GAA-RICH TRIPLET REPEATS IN THE 3' UNTRANSLATED REGIONS OF 2 GROUPS OF MESSENGER-RNAS ENCODED BY THE RAT POLYMERIC IMMUNOGLOBULIN RECEPTORGENE

An unusual S1-nuclease sensitive microsatellite (STMS) has been found in the single copy, rat polymeric immunoglobulin receptor gene (PIGR) terminal exon, In Fisher rats, elements within or beyond the STMS are expressed variably in the 3' untranslated regions (3'UTRs) of two 'Gro ups' of PIGR-encoded hepatic mRNAs (plg-R) during liver regeneration. STMS elements include neighboring constant regions (a 60-bp d[GA]-rich tract with a chi-like octamer, followed by 15 tandem d[GGA] repeats) that merge directly with 36 or 39 tandem d[GAA] repeats (Fisher or Wis tar strains, respectively) interrupted by d[AA] between their 5th-6th repeat units. The Wistar STMS is flanked upstream by two regions of ne arly contiguous d[CA] or d[CT] repeats in the 3' end of intron 8; and downstream, by a 283 bp 'unit' containing several inversions at its 5' end, and two polyadenylation signals at its 3' end. The 283 nt unit i s expressed in Group I plg-R mRNAs; but it is absent in the Group 2 fa mily so that their GAA repeats merge with their poly_A tails, In contr ast to genomic sequence, GGA triplet repeats are amplified (n greater than or equal to 24-26), whereas GAA triplet repeats are truncated var iably (n less than or equal to 9-37) and expressed uninterruptedly in bath mRNA Groups, These results suggest that 3' end processing of the rat PIGR gene may involve misalignment, slippage and premature termina tion of RNA polymerase II, The function of this unusual processing and possible roles of chi-like octamers in quiescent or extrahepatic tiss ues are discussed.