IDENTIFICATION AND FUNCTIONAL-SIGNIFICANCE OF A NEW CLASS OF NON-CONSENSUS-TYPE PLASTID PROMOTERS

The promoter regions of most plastid transcription units have been rep orted to consist of prokaryotic -10 and -35-like consensus sequences. However, a few promoters with no homology to the consensus elements ha ve also been characterized. A novel class of non-consensus plastid pro moters - designated as non consensus type II (NC-II) promoters -that e ffect low-level constitutive expression of respective genes in photosy nthetic as well as nonphotosynthetic plastids is described in this pap er. The abundance of NC-II promoter-derived transcripts remains unalte red even when light-grown seedlings are shifted to the dark. In contra st, transcripts from -10 and -35-like elements containing consensus ty pe (CT) promotes accumulate to high levels in chloroplasts as compared with non-photosynthetic plastids of roots and cultured cells. Moreove r, accumulation of these transcripts is greatly affected by light. The inhibition of plastid protein synthesis has no apparent effect on the abundance of the NC-II transcripts whereas levels of CT transcripts a re greatly reduced. In vivo tagetitoxin (a plastid transcription inhib itor) treatment also reduces the levels of CT transcripts with no appa rent inhibitory effect on the accumulation of NC-II transcripts. The a ccumulation of transcripts from both classes of promoters, however, is reduced when cytoplasmic protein synthesis is inhibited by in vivo tr eatment with cycloheximide. The results are suggestive of the possible existence of at least two distinctive systems for the synthesis and/o r maintenance of plastid transcripts which differentiate between two c lasses of transcripts in a promoter-type specific manner.