FAMILY OF SHORT, INTERSPERSED REPEAT SEQUENCES AT THE 5' END OF A SET OF DICTYOSTELIUM SINGLE-COPY MESSENGER-RNAS

M4 is a recombinant plasmid containing Dictyostelium nuclear DNA. The restriction map has been determined (K. Kindle and R. Firtel, manuscript submitted) and the four major fragments have been subcloned in pBR322. M4 is >90% single-copy; however, it does contain a short (300 bp) sequence repeated ∼100 times in the Dictyostelium genome. The short repeat sequence is interspersed between single-copy regions. 1-1.5% of total vegetative poly(A)+ mRNA hybridizes to the repeat, but only 10 % of the hybridization is resistant to low levels of RNAase. The mRNA is heterogeneous in size and 90 % of the mass is complementary to Dictyostelium single-copy DNA. We have also shown that one of the single-copy regions adjacent to the repeat is complementary to a low abundance class mRNA (0.01% of total mRNA). Sandwich hybridization experiments show that unlabeled RNA that hybridizes to this single-copy region will also hybridize 32p repeat. Exonuclease studies indicate that the repeat and single-copy regions are part of a single transcription unit which encodes a 1.2 kb mRNA. The 5′ end of this mRNA is complementary to the short repeat sequence and the remainder of the mRNA is transcribed from single-copy DNA. Furthermore, RNA excess hybridization to 32P-labeled separated strands of the repeat suggests that the repeat sequence is transcribed asymmetrically into poly(A)+ RNA. We have proposed a model in which this repeat sequence is adjacent to ∼100 different single-copy genes and is transcribed to produce different mRNA molecules carrying the common repeat sequence at the 5′ end. © 1979.