A ribosomal protein gene (rpl32) from tobacco chloroplast DNA is transcribed from alternative promoters: similarities in promoter region organization in plastid housekeeping genes.

Multiple transcriptional start sites have been identified in the tobacco plastid ribosomal protein gene rpl32 by RNA mapping and in vitro capping techniques. A promotor with a canonical -10 Pribnow Box (P1) produces a major transcript in leaf chloroplasts. Transcription is also driven from additional promoters in non-photosynthetic plastids from heterotrophically cultured cells (BY2 line). Among them, a second promoter located downstream (P2) generates the most prominent transcript in this type of cell. The absence of typical plastid promoter motifs upstream of this site and the higher steady-state level of the P2-derived transcript in BY2 cells suggest a distinct modulation of transcription. Mobility shift experiments also seem to indicate the existence of differences in protein-DNA binding between both kinds of plastids with respect to a DNA fragment including the sequence upstream from the P2 starting site. The structure of the rpl32 promoter region is discussed in relation to that of other plastid housekeeping genes encoding elements of the genetic machinery.