Strong functional GC pressure in a light regulated maize gene encoding subunit GAPA of chloroplast glyceraldehyde-3-phosphate dehydrogenase: lmplications for the evolution of GAPA pseudogenes

Summary. The light-regulated nuclear gene en-coding subunit A of chloroplast glyceraldehyde-3-phosphate dehydrogenase (subunit GAPA, gene Gpal) from maize is extremely G+C rich (67 % in codons). The genomic surroundings ofthis gene have been characterized together with the sequences of two strongly conserved Gpa pseudogenes i olated from a genomic maize library by differential cDNA hybridization. The comparisons show that the high G+C content of the maize gene is maintained in-dependently of the surrounding noncoding se-quences, which are G+C poor (42%), and only as long as the gene encodes a functional protein. After nonfunctionalization, Gpa pseudogenes rapidly loose G+C mainly due to enhanced turnover of CpG and CpXpG methylation sites. These results uggest that the maize Gpal gene is under strong functional GC pressure, due to constraints (CpG island) probably exerted at the transcriptional level. They also in-dicate that Gpa pseudogenes are methylated and that methylation was either the cause or the im-mediate consequence of their nonfunctionalization. It can be concluded further that the progenitor of pseudogenes 1 and 2 was a second functional Gpa gene (Gpa'), which, after duplication, accelerated in evolutionary rate due to relaxation of selective con-straints. This is in agreement with the neutral theory of evolution. Comparison of Gpa intron sequences reveals a gradient of divergence: the more 3 ' the position of an intron the more its sequence has di-Offprint requests to: R. Cerff verged between the three Gpa genes. A speculative model is presented explaining these observations in terms of a homologous recombination fgenes with their reverse-transcribed pre-mRNAs