[Functional systems and blockwise evolution: the relationship between fixation of new domain copies and the present structure of the associations in the functional system].

The distribution of paralogous domains in relation to the boundaries of functional systems (FSs) is examined. It was found that the frequencies of particular domain types in genes for the hemostasis and complement FSs by far exceeded the frequencies expected on assumption of their random distribution in the genome, i.e., the domains were not randomly distributed in relation to the FS boundaries. For instance, it was shown that approximately 50% of the total mRNA of genes for the hemostasis and complement FSs encodes 20 domain types repeated on average 2.7 (from 2 to 115) times. Thus, the present structure of the FS associations plays a key role in the formation of new associations in the system evolution. Possible causes and mechanisms of the accumulation of paralogous genes and domains in these systems are discussed. The distribution asymmetry may be explained by the systemic character of the organization (system connectivity). Since any structural innovation must be included in the scheme of the present associations, the new protein must contain at least one functional site complementary to sites of the molecules already functioning in the system. The mechanism of preference of "own" domains probably consists in fixation via selection of the shortest among many alternative possible formation pathways of the new functional structure. This mechanism must promote the accumulation in the FS of copies of already functioning structures (genes, domains) that can relatively rapidly adapt for performing the new function.