Ribosomal RNA Synthesis in Soybean Suspension Cultures Growing in Different Media

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Abstract
The transcription of ribosomal RNA was studied in suspension tissue cultures of G. max L. Merr cv. Mandarin cells (SB-1). A large precursor molecule was synthesized which contains RNA homologous to the 25 and 18S cistrons. Transcription was from 1 strand and apparently started adjacent to the 18S cistron and proceeded through the 18S DNA, a < 0.6-kilobase transcribed spacer and the 25S cistron. A nontranscribed spacer region was identified. When cells grew rapidly in sucrose (24 h doubling time) they contained 7 times as many ribosomes as when they grew slowly in maltose (200 h doubling time). Upon transfer from maltose to sucrose, cells began to accumulate ribosomes at a rapid rate (80-fold more rapid synthesis than in maltose medium) within 2 h at 33.degree. C. The 2 h lag is due in large part to a longer processing time during which newly synthesized RNA is packaged into ribosomes. The increase in transcription rate may occur within a few minutes of the transfer to sucrose.