[Study of the minimum frequency of phase 2 introns]

A study has been made on the distribution of introns of different groups with respect to phases. It has been established that group II introns and nuclear introns have a similar minimum of relative frequency of phase 2 introns. Since the phase of introns is an extremely conservative index, the observed minimum is a trace of relic evolutionary processes. A sample of all known group I introns proved to be insufficient to obtain a valid characteristic of phase distribution. It has been shown that the picture observed in the phase distribution cannot be explained on the basis of the mobile properties of introns only. One of the probable explanations for the intron phase distribution nonuniformity is the process of exon shuffling. An assumption is made that group II introns originated at the early stages of evolution and were involved in the process of exon shuffling.