The complete sequences of 12 plastid genomes have recently been determined. This has revealed that in addition to known genes involved in plastid gene expression and photosynthesis, there are numerous open reading frames encoding genes of unknown function. Several of these genes have been inactivated in Chlamydomonas reinhardtii and tobacco by biolistic chloroplast transformation. These studies — coupled with molecular, biochemical and biophysical analysis — have revealed the existence of novel subunits and factors required for the accumulation and optimal functioning of photosynthetic complexes. Other plastid genes have also been identified that are essential for cell survival, but whose function remains to be determined.