The chloroplast genome contains on the order of 100 genes; the proper expression of which are necessary for photosynthesis and chloroplast development. A critical step in the expression of a number of chloroplast genes is RNA splicing; the process where internally-located sequences (introns) are excised, and coding sequences (exons) are precisely religated. Comparative sequence analysis suggests that most chloroplast introns can be classified as Group I or Group II, however, there is very little direct information on the mechanism of RNA splicing in chloroplasts. Here, results are presented which indicate that (1) at least one nuclear gene product is required for efficient splicing of the rrn 23S intron of Chlamydomonas in vivo and, (2) 23S preRNA is capable of self-splicing in vitro. Preliminary evidence is also presented which suggests that introns of the psbA gene of Photosystem II and other unidentified Group I introns in Chlamydomonas are capable of self-splicing. Studies are proposed to elucidate, as far as possible, the mechanism and control of splicing of chloroplast Group I introns. Both in vitro (self-splicing) and in vivo (genetic) approaches will be utilized. Understanding the mechanism and control of RNA splicing in chloroplasts is important for at least two reasons. The first is due to the fact that plants are the primary source of biologically useful energy on this planet. Plants obtain energy through the process of photosynthesis, which is dependent, at least in part, on the expression and regulation of chloroplast-encoded genes. Secondly, there are many unanswered questions concerning the origin, function, and splicing of introns in eucaryotes; the role of trans-acting factors are of particular interest because of their probable role in regulating intron-related processes. Although the chloroplast genome has coevolved with the nuclear and mitochondrial genomes of plant cells, it has a distinct origin and many unique characteristics. Therefore, could be a useful source a new basic information on cellular and molecular processes that involve introns.
