COX assembly and the mitochondrial cycle.

COX (cytochrome c oxidase) assembly is a highly regulated multi-step
process involving discrete short-term intermediates, S1, S2, S3
[23]. The table describes the known
components involved in COX assembly and the relevant properties of
the corresponding mRNAs. MLR (Mitochondria Localization of
nuclear-encoded mRNA) characteristics are from [10];
classes indicate whether the mRNAs are translated on Puf3p-dependent
(class I), mitochondria-linked (class II) or on free polysomes
(class III). Phases A to C correspond to the early time-window of
the mitochondrial cycle. The first step of COX assembly is the
site-specific translation of the mitochondrially encoded COX
subunits. For instance, COX1 mRNA is translated under the control of
the translation regulators MSS51 and PET309 (both are class I mRNAs
present during phase A, Dataset S3). The second step is
the addition of Cox5p and Cox6p; note that Cox5p is the only
structural subunit belonging to phase A, consistent with its role in
early assembly step [25]. The
last step is the addition of the rest of the nuclear-encoded
subunits (shield proteins). These two last steps require the
presence of the assembly factors. Note that most of the assembly
factor transcripts appear during phase A, whereas the shield protein
transcripts are present during phase B. In addition, assembly factor
transcripts are localized to the vicinity of mitochondria (MLR,
class I) and depend on Puf3p for this localization. Shield protein
transcripts are translated on free polysomes (MLR class III) and
have no Puf3p binding site.