The trnK gene endocing the tRNALys(UUU) has been located on mustard (Sinapis alba) chloroplast DNA, 263 bp upstream of the psbA gene on the same strand. The nucleotide sequence of the trnK gene and its flanking regions as well as the putative transcription start and termination sites are shown. The 5' end of the transcript lies 121 bp upstream of the 5' tRNA coding region and is preceded by procaryotic-type "-10" and "-35" sequence elements, while the 3' end maps 2.77 kb downstream to a DNA region with possible stemloop secondary structure. The anticodon loop of the tRNALys is interrupted by a 2,574 bp intron containing a long open reading frame, which codes for 524 amino acids. Based on conserved stem and loop structures, this intron has characteristic features of a class II intron. A region near the carboxyl terminus of the derived polypeptide appears structurally related to maturases.

No abstract

The mustard chloroplast gene rps16 is split by an 887 bp group II (or III) intron. Three RNA 5' ends upstream of the rps16 coding region define both the transcription start site and two RNA processing sites. The DNA region preceding the start site contains a procaryotic-type "-10" promoter element, but not a typical "-35" element. One single RNA 3' end has been detected downstream from the rps16 coding region, but it is not in close proximity to any inverted repeat that might serve as a termination signal. Northern analysis has revealed several rps16 transcripts ranging in size from 1.6 kb to 0.5 kb. During seedling development, transcript levels show an initial increase and then remain constant without much difference between seedlings grown under light or in the dark.

Transcript levels of two plastid genes were investigated during early seedling development of mustard (Sinapis alba L.) until 96 h after sowing. The two genes, which are closely linked and have the same polarity, are the psbA gene encoding the Mr 32-35000 herbicide-binding QB-protein of photosystem II and the trnK gene encoding plastid tRNA(Lys) (UUU) and potentially an intron-derived maturase-related protein. By using Northern and dot blot hybridization techniques with sensitive RNA probes, the 1.2 kb psbA transcript was found to be present in low amounts during the initial phase of seed germination. Thereafter, it increases in concentration both in light- and dark-grown seedlings until approximately 48 h after sowing. A further increase in psbA transcript concentration during the subsequent phase until 96 h was observed in light-grown, but not in dark-grown seedlings. The 2.8 kb trnK transcript is one to two orders of magnitude less abundant than the psbA transcript throughout the time period investigated. The concentration of this transcript is light-independent and shows a transient peak level at around 48 h, i.e. at the onset of light-enhanced accumulation of the psbA transcript.

The mustard chloroplasts genes psbK and psbI are co-transcribed, giving rise to precursor transcripts of several size classes, which are processed to the monocistronic mature RNAs. The psbK and psbI coding regions are flanked by the two tRNA genes trnS-GCU and trnQ-UUG on the opposite DNA strand. Transcript mapping indicates that the (primary) psbK-psbI transcript overlaps the complete trnS-GCU and trnQ-UUG transcripts. The transcription start site of the psbK operon appears to overlap that of the rps16 gene. During seedling development, the psbK and psbI precursors and mature transcripts all become detectable between 30 and 48 h after sowing and then remain at constant levels without much difference either in light or in darkness.