NUMBER AND STRUCTURE OF BALBIANI RING 75-S RNA-TRANSCRIPTION UNITS IN CHIRONOMUS TENTANS

The number and structure of the Balbiani ring (BR) 75 S RNA transcription units in Chironomus tentans have been studied in DNA-RNA hybridization experiments. Using DNA excess hybridization it was shown that BR 2 RNA is complementary to highly repetitive DNA sequences (4000 to 5000 times per haploid genome) as well as to sequences with a low degree of repetition (6 to 8 times). Available information indicates that BR 2 RNA represents growing RNA molecules from a single 75 S RNA transcription unit, and it is therefore concluded that the two types of sequences are present in one and the same 75 S RNA transcript. BR 1 RNA, known to consist of nascent 75 S RNA with different nucleotide sequence from that of BR 2 RNA, was also found to be complementary to highly repetitive DNA sequences (1300 to 1600 times) as well as to sequences with a low degree of repetition (13 to 16 times). Cellular 75 S RNA was analysed in order to determine the number of the corresponding BR transcription units. As demonstrated by in situ hybridization, cellular 75 S RNA consists of RNA generated in BR 1 and BR 2. The hybridization properties of cellular 75 S RNA with excess DNA were found to be in agreement with such a conclusion. The cellular 75 S RNA was used to titrate the amount of complementary DNA. A saturation level of 0.043% was reached, corresponding to between four and five 75 S RNA transcription units. Since the distribution of these units between BR 1 and BR 2 is not known, it is concluded that each BR may contain from one to four 75 S RNA transcription units per chromatid. The major part (60 to 75%) of each unit consists of highly repeated sequences and the rest consists of sequences present once or a few times per unit. The functional significance of the repeated structure of the 75 S RNA transcription units is discussed. © 1979.