CONTROL OF REPLICATION OF THE LACTOBACILLUS-PENTOSUS PLASMID P353-2 -EVIDENCE FOR A MECHANISM INVOLVING TRANSCRIPTIONAL ATTENUATION OF THEGENE CODING FOR THE REPLICATION PROTEIN

The synthesis of plasmid DNA and of RNA encoded by the replication pro tein gene (rep) of plasmid p353-2 of Lactobacillus pentosus was studie d for the wild-type plasmid and for a mutant plasmid with a deletion i n the 5' untranslated region of the rep gene. Plasmid p353-2 codes for two countertranscript RNAs (CT-RNA) of approximately 75 and 250 nucle otides transcribed from the 5' untranslated region of the rep gene, in opposite directions. In a mutant plasmid with a deletion of the promo ter and part of the CT-RNA-encoding sequence which shows a 5- to 10-fo ld increase in copy number compared to the wild-type plasmid, no CT-RN A could be detected. In the wild-type plasmid more than 90% of transcr iption initiated at a promoter upstream of the rep gene is prematurely terminated to form a 190 nucleotide truncated RNA, whereas in the mut ant plasmid nearly all transcripts reach a size (1100 nucleotides) cor responding to that of the rep gene. A model is presented for the role of CT-RNA in control of plasmid replication, similar to that previousl y presented for the staphylococcal plasmid pT181, involving a mechanis m of transcriptional attenuation of rep RNA at a site just upstream of the rep gene.